Had a little downtime so thought I would revisit this. Made no difference.
Still hangs on every reboot, have to force reset.
No iGPU access.
Had to rollback to 6.12.4, again. And everything was back to working immediately.
However, noticed this time on reboot I had a bunch of extra random stuff appear onscreen, followed by the message
"not automatically fixing this"
Research suggested running chkdsk on the USB. I did that, but no errors found. Plugged back in to the same USB port and rebooted. However now it seems to be stuck at "Verifying bzfirmware checksum...", but it does let me login to the GUI.
Further research suggested copying over the original bz* files. Did that, doesn't get stuck on verifying anymore, but the random stuff and "not automatically fixing this" remained.
Now found this thread https://forums.unraid.net/bug-reports/prereleases/there-are-differences-between-boot-sector-and-its-backup-r1745/ and followed it. Seems to have fixed that issue.
TL;DR back on 6.12.4 and going to leave it alone at this point. Once something newer than 6.12.6 comes out I will wait and see if others report success before I even try.