OK, I removed the 'quiet' flag and added 'textmode' and 'ssh" flags to syslinux.cfg on the internal SSD, and it boots 12.2 successfully to the shell. Pastekodi yields: https://paste.libreelec.tv/smooth-squirrel.log
I then removed 'textmode', and while watching it boot I can see it starting sshd, but again it starts the graphics subsystem and gets into a loop. Interestingly I cannot ssh into it at this point. What's more, there's no DHCP lease on my server.
So while it's not a reboot loop, whatever it's choking on seems to prevent it from obtaining an IP address. Hence, I cannot get in to it to generate logs via pastekodi. If I boot it via USB and ssh into it, can I grab the logs from the failed graphics shell boot and put them somewhere?