An update to my original post
here. Tldr: I selected the MKII's (w/Dekoni Fostex X00 sheepskin elite pads!) as my favorite out of all of these HP's listed below.
After considerable time and effort (and return shipping fees haha), I've selected the Fostex MKII w/Dekoni pads as my closed-back for home use. In the end, it came down to a head-to-head battle between these and Denon d9200's. The MKII's won out.... but just barely. The d9200's are a great HP too. However I had to choose one. I should mention, there's no way I would've chosen the MKII's with the stock pads... they are terrible.
I feel it's worth noting, I'm very surprised that after going through all of these HP's - a lengthy process which first entailed reading multiple online reviews, then buying, personally testing and listening to each one, then returning those I didn't like - I never did find
exactly what I was looking for. I had three main criteria (as I'm sure we all do with closed-backs): isolation, comfort, and, of course, sound. Not one of these hit all three. Not one! That was surprising to me. So after all this work, I'm still left feeling a little disappointed. That said, my ears do enjoy the MKII's sound the best out of all of these, so I feel good about that.
If I could build a HP, I'd combine the MKII with the Dan Clark (Mr.Speakers) Ether C Flow 1.1. The comfort and isolation on the Ethers was wonderful. I ranked them #1 in those two categories vs all of the HP's I tried (they were tied #1 with Focal Stellia for isolation). However, since sound trumps all, I went with the Fostex.
In fact, I tried switching the Ether pads over to both the d9200's and the MKII's. I also bought about 10 different pads to try for both of these HP's as well. That was an effort all in itself! In the end, stock pads on the d9200 are the best for that HP, and Dekoni Fostex X00 sheepskin elite are best on the MKII's. The stock pads on the MKII's should be ashamed of themselves.
Here is the complete list, sorted by price, of all the ones I tried. While I did take notes on each one, writing my own reviews and posting here would take forever. Since I'm a newb reviewer, and there's tons of other reviews on all these out on the interwebz, I'm going to skip that part.
(on a side note, have you ever noticed the sometimes
really hard-to-understand language used in HP reviews? For the most part, reviews can be incredibly helpful. However, from time-to-time, I'd read a review with language that just made me say, What does that mean!? Case in point, I came across this phrase a few weeks back and had a good chuckle, "polite transients along with soft and not too distinct microdynamics". Maybe you guys know what that means, but I sure as hell don't!

)
Sony MDR7506
Audio-Technica M50x
Sennheiser HD 569
Audio-Technica MSR7
Audio-Technica MSR7b
Audio-Technica M70X
Audio-Technica SR9
Focal Elegia
Ultrasone Signature DXP
Audio-Technica ATH-WP900
Beyerdynamic T5p 2nd Gen
Denon AH-d7200
Dan Clark Audio AEON 2
AUDEZE LCD-XC
Audio-Technica ATH-AP2000Ti
Denon AH-d9200
Mr.Speakers Ether C Flow 1.1
Fostex TH-900mk2 Premium --- WINNER
Sony MDR-Z1R
Audio-Technica ATH-AWKT Kokutan
Focal Stellia
So, FWIW to anyone reading this, that's where I landed. Hopefully this might help someone else. This is really a great place, I've learned a lot from reading all these posts and reviews. Thanks to all of you who unknowingly contributed to my deep-dive into the world of closed-backs.
Next up on task-list is trying balanced cables. Then maybe dac/amps.