You should keep in mind pistols only do piercing damage and there are quite a few enemies immune or highly resistant to piercing, so you'll either want Eccea's Arcane Blaster for them or have a secondary weapon like a bow (Essence Interrupter is awesome, get it in Port Maje). Or your fists if you go devoted.
So I'll discuss some specific builds but first a rundown of the subclasses
Devoted - gets intuitive from disciplined strikes, +2 pen and some hit to crit with chosen weapon (also fists), effectively +20 accuracy with Conqueror stance, good survivability with rapid recovery and unbending, some graze to hit with confident aim, quick switch. Accuracy penalties for non-chosen weapon.
Black Jacket - gets intuitive from disciplined strikes, effectively +20 accuracy with Conqueror stance, decent survivability with rapid recovery (weaker than other fighters) and unbending, some graze to hit with confident aim, can switch weapons instantly with quick switch.
Troubadour - Sure-Handed Ila gives -20% reload time, lots of summons and CC, can give self Energized with Their Champion Braved The Horde Alone for +2 pen and interrupt on crit.
Cipher - Psychovampiric Shield is effectively +10 accuracy and +5 resolve, Borrowed instinct gives +20 accuracy +20 all defenses, Hammering Thoughts gives +1 penetration, many CC and party buff spells as well as direct damage. Generic cipher is sometimes the best subclass.
Soul Blade - melee focused cipher, gets soul annihilation, could work with devoted, switch to fists when focus is full and use soul annihilation for massive raw damage
Ascendent - weak while focus not full, once "ascended" you do lots of damage and have infinite casts (for a while anyway)
Ranger - TONS of +accuracy abillities (+25 to +35), -20% reload time, pistol shots bounce, and an animal companion to take some of the heat off you
Sharpshooter - Slight recovery (10% IIRC) and deflection (-10) maluses, gets bonus +1 penetration for targets nearer 4m and +hit to crit for farther targets.
Ghost Heart - ranger without permanent companion, have to summon, benefit is you can't get bonded grief
Arcane Archer - an interesting caster/ranged hybrid, has some very cool imbued ammunition effects but they tend to cost a lot of bond. You get -5 accuracy when firing non-imbued ammunition (including on eccea's arcane blaster which fires imbued ammunition)
Rogues - Have lots of +damage modifiers assuming you can apply one or more afflictions. They have bad accuracy so you'd want to supplement with another class with high accuracy, like a scout or mindstalker. I haven't played rogues much so I don't mention them in the below builds.
Monks - Gunfu monks do better with mortars or blunderbuss, IDK about using them with pistols, though here is a build if interested (paladin/helwalker). I guess it stacks a lot of +damage from helwalker, and enduring dance gets you +12 accuracy, but this is weak compared to what you can get from other classes IMO.
https://steamcommunity.com/sharedfiles/filedetails/?id=1409067909
Casters, paladins, barbarians. Role-playing it doesn't seem like they'd use pistols does it? Also not a lot of synergy I'm aware of with druids, priests, or wizards. Paladins have a bit but not as much as other classes IMO. Most of barbarian's cool stuff requires a melee weapon (carnage and carnage related talents), also their accuracy is bad, but they do get a ton of action speed and damage buffs, could possibly work in some multiclasses.
And now I'll ramble about various combinations of these classes and subclasses
Devoted/troubadour is great if you don't have a chanter in your party already. Troubadour/anything tends to be awesome actually. Run with brisk recitation on so you can use invocations more often. Also, with devoted/troubadour, you can keep a wall of skeletons running away from you with the tier 7 Many Lives Passed By, and enemies will attack these skeletons which provides you cover in a manner similar to a ranger but more effective because you don't care if the skeletons die. Devoteds like all fighters get disciplined strikes for intuitive which is really, really good.
Devoted/cipher for big accuracy buffs (psychovampiric shield is effectively +10 vs the enemy targeting, and borrowed instinct another +20) and penetration bonuses (+2 from devoted, +1 from cipher's hammering thoughts), can run devoted/ascendent if want to focus on the spellcasting side. Could also run a devoted/soul blade, pick up monastic unarmed training, and switch to fists to use soul annihilation. Devoted also gets the +2 pen and hit to crit conversion for fists. The way to play ciphers depends on subclass, soul blade is mostly about using soul annihilation, besides that you just keep your accuracy buffed with PVS and BI and buff party members with tactical meld, pain block, resolute shield. With an ascendant it's more about spell-casting, so you'd pick CC stuff like Mental Binding, Silent Scream, and damage spells like Disintegration, can also hand out brilliant to party members with Ancestor's Memory. Pistols won't develop focus quite as well as some other weapons but as long you aren't trying to set records they're fine.
Black jacket is a decent subclass if you want to be able to swap guns quickly. Works well with cipher alpha strikes. But devoted gets the bonus pen and hit to crit so if you know you want pistols it is hard to beat.
Devoted/sharpshooter would have top-tier accuracy and penetration. Take Conqueror Stance and Weapon Mastery on fighter side, plus Disciplined Strikes (or Tactical Barrage) for an effective +20 accuracy on fighter side, +2pen on fighter side, and whatever the hit to crit is (15?). Then on ranger side sharpshooter gets +1 pen against close targets and some more hit to crit for distant targets. Take Marked Prey / Marked for the Hunt, Marksman, Gunner, Protective Companion / Stalker's Link, Driving Flight, and Survival of the Fittest. With these you get effectively +15 accuracy against marked targets, +25 against marked targets your companion is attacking, +35 against marked targets your companion is attacking that have < 50% health, -20% reload time, and +1 weapon bounces. In addition to the devoted side. Sharpshooter does have a slight downside in a small recovery time bump and deflection malus, so devoted/generic ranger is also perfectly acceptable, or devoted/ghost heart if you prefer, though you only get the stalker's link bonus if the companion is summoned and attacking what you're shooting at. Bears and boars are the strongest animal companions, bears for high armor and boars for high health pool and health regen. Take resilient companion for +2 armor and vicious companion to make them a little less bad at fighting.
If you want super top tier accuracy, a ranger/cipher is the best choice. This build will crit more than any other as you should have stacking +55 to +65 accuracy as long as borrowed instinct is up and you cast psycovampiric shield on things. Can get that up to +60 to +70 accuracy if you cast tactical meld on your animal companion. Cipher goes really, really well with ranger because you can use the "ally only" spells like pain block, resolute shield, tactical meld, amplified wave, stasis shield etc. on your animal companion (some of these buff you as well, like tactical meld and echoing shield). And the stacking accuracy bonuses. Ciphers also get bonus damage while soul whip is active, +20% with draining whip. A devoted would have effective +45 to 55 accuracy, some graze to hit from confident aim, and comparitively +2 penetration and some hit to crit. But the devoted accuracy bonuses are passive and work on everything, you don't have to keep up borrowed instinct and psychovampiric shield.
A ranger/troubadour would also be interesting, and you'd have stacking -40% reload time from sure-handed ila and gunner. With rushed reload you'd attack quite fast indeed. (Though they don't stack linearly, you wouldn't have -90% reload time, it's some complicated double inversion thing).
Another idea is a single classed ranger. They are kind of meh in vanilla game, but if you use @Elric Galad Balance Polishing Mod (available at nexus mods) they are much better and become competitive with some of the best ranger multiclasses. What distinguishes the SC ranger are abilities like twinned shot (costs 1 bond in BPM), stunning shots, bonded fury (provides bond regeneration and buffs pet in bpm), and shadowed hunters (provides invisibility, healing, and intuitive buff). Shadowed Hunters works well with Ajamuut's Stalking Cloak which allows you to stun things from invisibility.
Monks are indeed very good but personally I never liked the idea of gunfu monks since monk fists are so good. Also gunfu monks work better with mortars.
With your pistol at legendary quality, you'll have effectively 13 pen with devoted / troubadour, 14 pen with devoted / cipher, and 14 pen with devoted / sharpshooter, 11 or 12 pen with ranger / troubadour or sharpshooter / troubadour, and 12 or 13 pen with ranger/cipher or sharpshooter/cipher. So what difficult you're playing is important. If you're on POTD upscaled high penetration is important so I'd probably go with the devoted/cipher, devoted/sharpshooter, or sharpshooter/cipher.
Devoted/sharpshooter will play the most straightforwardly. You shoot things, your animal companion attacks things. If you like the idea of caster hybrids, a devoted/cipher or sharpshooter/cipher is hard to beat. Devoted/troubadour is also quite good.
I've tested most of these builds to some extent. They'll all work. If you want to be a crit machine you'll want ranger/cipher, devoted/cipher, or devoted/ranger. And keep in mind the only way you're going to reliably crit with rushed reload on is to have tons of stacking accuracy. Personally I really like seers (ranger/cipher) but devoted/cipher would also be really good and a bit more straightforward since you aren't worrying about the pet dying. Devoted also has better pen and armored grace allows you to wear armors with no or little penalty (though robes are fine for ranged builds). But devoted is less flexible when you can't use the pistols. Another thing about cipher's borrowed instinct is it gives +20 all defenses which is insanely good, and psychovampiric shield gives +5 resolve so dumping resolve is safe.
Your stats for any of these builds should be roughly the same, since they all have similar functions. I like wild orlans and wood elves for ranged builds but you can use whatever you want. My approach is generally like this. Max perception, high or max INT, dump resolve, spend 20 to 25 points on might and con, and put the rest in dex. So for instance
MIG 14
CON 8
DEX 15
PER 20
INT 18
RES 3
You don't need to max dex since there are diminishing returns the more action speed you get, but you can max it if you want (just not at the cost of perception and intellect). You could dump CON even more if you're pretty sure you won't be getting hit, whether that's because your party can keep you in the backline or you have summons or an animal companion to protect you.
Critical items for a ranged build are the ring of the marksman (+4 accuracy, +1 pen), Sky Dragon Wurm (+3 accuracy, deflection, reflex, +10%damage with ranged) or Harley if you can't find him, ring of prosperity's fortune for 15% hit to crit (if you're rich), high harbinger robes for +10% damage and no recovery time (can wear devil of caroc BP with armored grace), ajamuut's stalking cloak in some cases, acina's tricorn, and gauntlets of accuracy.
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Finally, can you perma-paralyze with Thundercrack pistol? Hmm. I doubt it, but possibly. With maxed INT you can get the paralyze effect to last 4 to 5 seconds. A fire and reload takes (unmodified) 6.1s. You can reduce that quite a lot through combinations of Gunner, Sharpshooter Garb, Sure-Handed Ila, and Rushed Reload modal. Might be possible to perma-paralyze weak enemies, but it's going to be hard to crit against tougher enemies with rushed reload on (unless you're running one of the super high accuracy combos like a seer, psyblade, or hunter).
I'm going to do some testing and get back to you on this
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First of all, items that stack -recovery don't work as well as you'd hope. You can't get to -100. I stacked shootist from Acina Tricorn (-10), sure-handed ila (-20), rushed reload (-50), and with modified 30% attack speed get .6 attack time and 1.8 reload. Then added the effect from mahora wraps (another -20) to get recovery 1.6. So clearly there are diminishing returns, the base of 5 was only reduced to 1.6 with reductions totaling 130%. With max dex of 35 (barely possible if specced for it, plus items, boons and dex inspiration), the attack goes down to .6 with recovery 1.5 for an attack every 2.1s. With rushed reload off I have same .6 attack and recovery 1.9 for total 2.4.
Also, Thundercrack doesn't actually paralyze on CRIT, it CAN paralyze on CRIT. So if you crit vs. deflection, it then rolls against fortitude to see whether you miss, graze, hit, or crit. On my character with modified 30 INT, in the best case where I crit/crit, I get 5.2s paralyze vs a human, 6.1s vs a summon (it is modified by the enemy resolve as well as your INT). So if your accuracy is stacked high enough (can probably get to around 180), you could perma-paralyze weaker foes, though fortitude tends to be higher than deflection so even then you likely won't crit/crit that often.
If the target is already paralyzed, you can't see the roll for the paralysis, though in my limited testing once paralyzed he does seem to always get paralyzed if you crit. But I mostly get an additional 2 to 3s, so either I'm not critting on the paralysis roll or the mechanics are different when already paralyzed. This was a high level enemy (Tumako the Tanager in SSS) and he stayed paralyzed once I got the first crit/crit, until near death when I missed the crit roll. My character has super high accuracy and lot of hit to crit.
I'd definitely recommend ranger as half of your combo if for no other reason than your attacks bounce. So thundercrack can for example paralyze two characters at once, as you see happens below.
My test character is an arcane archer / troubadour. Arcane archer is pretty interesting. You do get -5 accuracy when using non-imbued ammunition, but my accuracy is so high this isn't that big a deal. The Pull of Eora shots are particularly good. They work well enough with Thundercrack pistol, you can see here I have multiple characters paralyzed and the rest flopping around doing nothing (my bear is just standing there since he got hit with thorny roots so my accuracy isn't even maximized). This kind of thing would probably work better with mortars though, so your mortars would hit the enemies clumped together by the pull. For something really broken you could make an arcane archer / monk (normal in vanilla game, shattered pillar with community patch) and use Stunning Surge on pulled groups with mortars. You will usually crit at least once and get the surge cost refunded, as well as gaining more wounds.
Eccea's Arcane Blaster is really good also. The Imbued Ammunition types have some cool effects, and it does raw + crush/corrode, and that combination is generally going to be resisted very poorly, so in cases where Thundercrack pistol isn't working, the blaster should do it.