1) Kernel 6.16 caused two crashes in half a day (but this seems to be related to the processor and not the GPU).
2) I didn't notice it at first, but version 6.12.38 is provided in addition to version 6.12.41, and the former is considered a little more stable, so I'm testing that one for now.
3) OK, so I'll try this dichotomous process to identify the culprit. It may take a long time!
4) In addition to the lockups, I see a lot of messages like this (I don't know if it's anecdotal):
August 22 18:45:20 station1 firefox-esr.desktop[6173]: [GFX1-]: Unable to clean up GL_RENDERER “KABINI (radeonsi, , ACO, DRM 2.50, 6.12.38+deb13-amd64)”.