Thanks!You could try booting into safe boot to see if that at least lets the computer start up.
You could also boot into recovery and select the "Restore from Time Machine" option, then select your internal disk as the source. If there's a snapshot from before the 11.1 update, select that and it should revert the computer.
Alternatively, I'd try starting from Internet Recovery and reinstalling what should be 11.0.1, if I had a good backup of my data. Reinstalling shouldn't affect your files but I haven't tested this with Big Sur so I don't know if it'd be successful.
same issue in safe mode.
Internet recovery, reinstall says “the volume cannot be downgraded” when I select the disk.
option-command-r gave me two errors so far. -2003F and -2004F
shift-option-command-r gave me -2100F