Dps-wise and speaking about "self-contained builds" I guess there's not much that comes close to the described Streetfighter/Troubadour with wisps, but it doesn't have great accuracy.
So I'd say Ranger/Troubadour with Marked for the Hunt, Marksman, Driving Flight, Gunner and so on and Sure-Handed Ila + Aefyllath Ues Mith Fyr, using the Red Hand. Great reloading speed, great accuracy, good versatility (due to invocations, esp. summons), good dmg per shot (burning lash at all times).
But it's true that you only need a chanter in the party for that. If there is one then including the rogue class is great. For example:
If you don't enchant the Red Hand with Double Tap but the Knockdown effect instead, a Rogue/Ranger can be very good - if Arterial Strike is used a lot. I mean it's great anyway bc. of all the dmg bonuses paired with high ACC. But I mean it's even better imo if you can knock enemies away from allies with Arterial Strike and they will suffer greatly if they want to re-engage (they usually want that). With Driving Flights it's two enemies with one shot who will hobble around, bleeding out. It's very useful and does a lot of damage per shot, too. It's not as fast with reloading but good enough imo. You lose the double tap against vessels but I think it's worth it in this case.
If you have a chanter in the party you can still use the wisp trick and pick a streetfighter for the rogue/ranger.
For a solo arquebus user I think not much comes close to @Kaylon's Assassin/Tactician. It's posted here as a build somewhere with the mechanics explained.
Mechanically I'm not a big fan of the Devoted with an arquebus because he's at a disadvantage when he meets pierce resistant or immune enemies - or ones with an Arcane Veil (which are pretty frequent, esp. among the vessel and spirit categories).
Black Jacket is okay. I personally don't see a huge advantage when using 0-recovery weapon-set switching though because of the Red Hand arquebus. You buy up to 3 quick extra shots with increased micromanagement - I mean on paper it's worth it, but I find that one stops to care after a few encounters because the switching gets annoying. Especially when the fights last longer and longer it's not worth my time.
Conquerer Stance is very nice though. And with the Community Patch Weapon Mastery is nice, too.
Thematically a Sharpshooter/Devoted would make a lot of sense - and as long as you don't meet too many pierce immune foes (or just make your character sturdy enough to be able to punch those with Monastic Unarmed Training) it would also perform well. +3 PEN at closer range is pretty good - +6 when using Penetrating Strike: a lot of overpenetration I guess?