Some interesting findings:

First, I think I may have identified the problematic commit (or at least the
most-problematic one): d1a3e239a6016f2bb42a91696056e223982e8538
(drm/amd/powerplay: drop the unnecessary uclk hard min setting). I eventual=
ly
gave up on doing a normal bisect since so many of the commits between 5.0 a=
nd
5.1 were non-viable. Instead, I made a list of all the commits that touched
vega20-related files. I then started repeatedly picking out the non-tested
commit with the most related-sounding message, checking out the v5.1 tag, a=
nd
reverting the commit in order to test it as the culprit. When I revert that
one, my system boots reliably. I still see 133.0 watts of power draw, thoug=
h.

This brings me to the second thing: When looking through the commits, I not=
iced
that there were multiple commits that claim to prevent or reduce crashing in
high-resolution situations (one references 5k displays, another references =
3+
4k displays). I want to note that we all seem to have relatively demanding
display setups: Hameer has two 144hz 1440p displays, Tom B has two 60hz 4k
displays, and I have two 120hz 4k displays. Putting these together I decide=
d to
try unplugging one of my displays. Imagine my surprise when things booted
completely smoothly on a stock 5.1 kernel: glitch-free boot, *no powerplay
errors in the kernel log*, and 25 watts of power draw when usage is low. So=
 I
think it is safe to say that one "workaround" is to unplug a moni=
tor if you can
stand to work that way.

I actually have access to another Radeon VII so I may try running one per
monitor tomorrow.