And the Qubes 4.2 installer booted just fine. I guess that forum post was onto something there.
Discussion
My last ditch attempt to get my previous SSD to boot with the new motherboard has failed.
I thought maybe if I did a fresh install to a fresh SSD, it'd set up the UEFI variables in the BIOS to allow me swap the old SSD in there and boot up.
No such luck. So now I'm finally going to resort to restoring from a backup and then copying over the files that I manually rescued.