Ah, so you're wanting a spellcaster that can also tank. I tend to build mage/martial hybrids the other way, mostly casting a few buffs and then whacking things with a stick, vs whacking things with a stick then casting damage / CC spells.
I didn't realize the modals could be insta-switched like that. I mean it makes sense, but for some reason the animation on the modal refreshes every time you switch weapons (like turning it off to on) so I assumed it was actually cycling it off/on, but I checked and with quick switch you can in fact insta switch with the modals active which is pretty cool.
Your concept of quick switching to debuff fort/reflex/will is good, should be pretty effective and net you effective +25 accuracy when landing crucial spells
You probably want a club too, Kapana Taga is the best, does crush/slash and has good melee enchants (+engagement, flanking immunity etc.), and the club modal does -25 will.
Or you can stick with a morning star, the willbreaker morning star is really good for debuffs, -3 will on hit stacks 5 times, plus the fortitude debuff modal. But if you're doing black jacket might as well have dedicated club for when you need to quickly debuff will. In that case saru sichr may be preferable. You can get it early and it does a lot of corrode DOT.
And I guess yeah sun and moon is pretty dang good for this purpose of debuffing reflex and acting as a stat stick. I tend to prefer Sungrazer or Ball and Chain for the on-crit effects but if you're mostly casting hard to beat sun and moon.
I made a black jacket / blood mage with stats 8/14/15/19/18/3, if you don't want to dump resolve that much something like 8/11/13/19/18/8 would also work. Maxing dex isn't as big a concern because you can cast deleterious alacrity for a long lasting action speed boost (about 90s with the first stats). And we don't care a ton for resolve because of llengrath's safeguard and fighter bonuses helping out. Probably want at least some CON so can blood sacrifice, but with hylea's bounty and tough you can have a decent health pool even with middling CON.
There are very likely some cool weapon interactions of which I'm unaware. My knowledge in this area is kinda limited. Ones I do know: Bardatto's Luxury's Death and Taxes Hardy inspiration is easily procced on ally unconsciousness and has a long duration. You can proc it with essential/substantial phantoms also, switch to the mace and kill them at near death for the hardy inspiration which lasts around 2 minutes with high int
Also you might want one weapon slot with scordeo's edge and/or scordeo's trophy so you can open with that and get some -recovery time (or if lucky proc blade cascade and extend with wall of draining)
Griffin's Blade is really nice as a stat stick also. Can dual wield that in the offhand and get +10% damage with spells / 5 will or +10% action speed / 5 reflex, plus steadfast on 50% health once per encounter, in main hand use flail/club. Can dual wield it in multiple slots using "copy" function (little white square in top right of weapon icon).
My go to setup would probably be
slot 1: sun and moon + griffin's blade (for debuffs and casting)
slot 2: kapana taga + griffin's blade (for debuffs and casting)
slot 3: saru sichr or willbreaker (for debuffs and damage)
slot 4: scordeo's edge + scordeo's trophy or bardatto's luxury or whatever (for buffs and damage)
Or if you want to be tankier lethandria's devotion is a really nice medium shield that helps healing also. The large shield Akola's Apex Ward is super good but you have to hunt down a huana vegeance ship (wheneena eel's kiss) to get it, which requires tanking your rep with the huana really early (simplest way is kill everyone in takano's estate then hit the ocean), but it is top tier shield for making you tankier without sacrificing much accuracy due to blood rage effect (+10 accuracy on bloodied).
hope this helps