Things are becoming clearer...
I removed *all* non-free packages, namely:
ii amd64-microcode 3.20250311.1 amd64 Platform firmware and microcode for AMD CPUs and SoCs
ii intel-microcode 3.20250512.1 amd64 Processor microcode firmware for Intel CPUs
ii firmware-intel-graphics 20250410-2 all Binary firmware for Intel iGPUs and IPUs
ii firmware-intel-misc 20250410-2 all Binary firmware for miscellaneous Intel devices and chips
ii firmware-mediatek 20250410-2 all Binary firmware for MediaTek and Ralink chips for networking, SoCs and media
ii firmware-misc-nonfree 20250410-2 all Binary firmware for various drivers in the Linux kernel
ii firmware-realtek 20250410-2 all Binary firmware for Realtek network and audio chips
I now use the “official” linux-image-6.12.43+deb13-amd64 kernel and it no longer crashes at all, even though my system is slightly slower with the open source module. But in any case, it's perfectly usable for my work, as I don't need exceptional GPU performance.