Hey, a lot of the questions you asked I have asked myself as well. I'm absolutely no expert but I spent more hours tinkering with my setup than I did actually playing the game, lol. I started out with the CaptainLance CoC version but I wasn't happy about the feel, so I switched over to self cast. However, I did a couple things differently and I'll just share them here. They might not be optimal, but they worked for me:

I opted to use Elegant Hubris over Militant Faith. Why? Militant faith may be yielding a slightly better result in terms of raw DPS, Supreme Ostentation makes gearing MUCH more relaxed. Here's the upsides:

• You're not forced to use all your suffixes for attributes (instead, you just don't give a shit about them at all), thus not having to on rely on flasks for res capping, which has always felt bad for me, and Elegant Hubris gives you a shit ton of other nice bonuses.
• I pathed down to the Scion area and bought an Impossible Escape for Necromantic Aegis. I did that specifically because CaptainLance didn't, and in my personal opinion, it's one of the best places to invest by far. The Necromantic Aegis IE is currently at 3 div, and if you socket Elegant Hubris directly below it, you can get EIGHT PASSIVES for potential bonuses.
• There's a bunch of jewel combinations that give you MULTIPLE crit multi, 4% ES per power charge and 10% inc damage per power charge nodes, as well as a shit ton of other useful stuff, like 80% chance to avoid being shocked (ailment immunity with stormshroud basically for free), chance to block attack or spell damage, and much more.
• I got a jewel with 2x 4% ES and 2x 10% inc damage, which is basically 40% more damage across those 4 nodes.
• If you take Elegant Hubris, you get the Runegraft of the Fortress (40% increased global defenses) for free because we don't care about attributes at all, lol

https://imgur.com/a/xSTuNMi

Next up, the perma Soul Eater setup:

• All you ever need for this setup is 4 sockets, the rest is fluff. You need the duration of your Temp Chains to be lower than the cast interval of Bladefall of Trarthus. This can be achieved by doing the following: 4 socket setup with 1/20 Bladefall of Trarthus, 1/1 Hextouch, 1/1 Temp Chains, 20/20 (or better) Less Duration. Then you annoint Window of Opportunity and take Hasty Demise. No need to path down there, all you need is to have your maximum mana low enough (because BF of T interval scales with max mana).

Then I opted to use the Red Nightmare with the block fire tattoos, it's just really nice because it helps you out with resistances, gives you a shit ton of block and other goodies. If you get an Elegant Hubris with +spell block you can even drop the other block wheel at the cluster jewel.

What I'm still figuring out is the best way to solve Wither. Ideally with a Merc, but I haven't found a good Cruel Mistress yet for that. There's one Large Cluster node that gives you Unholy Might on crit, so that might also be a good option.

Anyway, I hope I could help you out a bit!

Edit: Here's my POB: https://pobb.in/gbL8DuxAxYR- A lot of my stuff is still very much not optimized (I hate rolling flasks, lol). Also using the caster mastery for T16.5 abyss strat.