Good news everyone! It was indeed the firmware (so a tip of the hat to
@siamhie) and the instructions provided by
@Charlie Brown to download and install the latest available were spot on. Doing some more digging today I found that despite being a new update, the latest firmware in the MX Repos are 6 months old (dated 20230625 vs the latest 20231211) - presumably that's driven by Debian, but I wrongly assumed repos would have the latest. What is interesting is looking at the linux-firmware log
https://git.kernel.org/pub/scm/linux/ke ... e.git/log/ I can see "Partially revert "amdgpu: DMCUB updates for DCN 3.1.4 and 3.1.5" on July 7, which might be related to the Dmesg errors I saw. There are several amdgpu updates since, probably mostly routine stuff...but nice to be at the latest.
I will take the advice of
@j2mcgreg and migrate to the Liquorix 6.6 kernel - I had been patiently been waiting for an official Debian one but after looking into that I learned there won't be one (which seems odd as it's an LTS kernel. Oh well).