Does it make make a difference if you boot without keyboard connected (I assume this is one of the Apple keyboards included in the imac).
The journalctl kernel.log is unfortunately truncated. can you instead post the output of sudo dmesg | grep pci for both the broken 6.10.9-401 and the working 6.10.6-401.
Changing the ID to boot from an older Kernel by default | grubby looks like it still contains accurate information on how to change the default boot kernel.