For the first time ever, I might finally have some very good news on this
issue! It will take several more days to confirm, then possibly another month
to pinpoint the exact option responsible. However it's possible I may have
found something that finally gets rid of the crash.

The issue appears to go away when playing Xonotic with those parameters:

export LIBGL_DEBUG=true LIBGL_NO_DRAWARRAYS=true LIBGL_DRI3_DISABLE=true
MESA_DEBUG=true MESA_NO_ASM=true MESA_NO_MMX=true MESA_NO_3DNOW=true
MESA_NO_SSE=true MESA_NO_ERROR=true MESA_GLSL_CACHE_DISABLE=true
MESA_NO_MINMAX_CACHE=true RADEON_NO_TCL=true DRAW_NO_FSE=true DRAW_USE_LLVM=0

I additionally disabled the cvar "r_shadows 2" which I forgot I had on for a
while now, as it enabled a shadowing system that might have itself been the
culprit.

With these two changes, I was able to clock up to 120 minutes of continuous
gameplay last night, followed by an outstanding 200 minutes today! That's over
2 respectively 3 hours with no system freeze whatsoever. I need to repeat this
test several times to be 100% sure there's not still some obscure chance of it
happening, but in any case there is definitely a major difference visible.