Fixed it! Turns out the ownership on those three drives for some reason had changed to root instead of me. I have no idea how that happened. Also one of the three drives had its mount point removed. I had to recreate the mount point and change the ownership of all three back to me. Now everything boots as it should.
Chaos. Just chaos…
Now all I have to do is restore all my tweaks and reinstall all my ops. Another fine day…
And then hope I don’t get any more memory-caused BTRFS errors…