OK, another update after another test, this time booting into a live system
without any graphical shell/login manager.

Here, manipulating /sys/class/intel_backlight/brightness does nothing to the
dGPU, so that part is clearly a user space bug.

However, a simple #lspci DOES crash the system in the manner described above,
so that part is still on amdgpu (and likely the root cause of the behaviour
described in my OP). Again, this is only an issue with dc=1 and not with dc=0.
Possibly related: https://bugzilla.kernel.org/show_bug.cgi?id=156341