New motherboard arrived and I have confirmed that was the problem.
Wouldn't boot into the existing M.2 drive, after many attempts at rescuing it (grub2-install, copying over .mod files, etc.), I got it almost restored.
If I manually load the grub2 config file, enter the FDE password, it gets right up to when X would normally load and then it resets. I'm not sure if it's some driver causing a fault or what.
I had a fairly recent proper backup, and I've copied some files to an external drives. So a full reinstall won't be too terrible if it comes to that. I shouldn't lose more than a week's worth of work at the most.
I have one more trick up my sleeve that might work, and then a last ditch effort that will require borrowing or buying some hardware. It'll probably be until early next week before I am back up and running.
