After a lot of tinkering, I have reached a stable state without any crash for more than a week now.
I removed all boot parameters I added over the last month.
I followed this description and downgraded linux-firmware to an older version from march:
System frequently crashing after GPU drivers update - #149 by Hans12
I’m now running Kernel 5.12 with linux-firmware 20210315.3568f96-3 and latest mesa.
Might be worth a shot if you still have problems.
The problem seems to be in the amdgpu driver which is in linux-firmware. Don´t forget to add linux-firmware to the pacman ignore list if you don´t want it to be updated to the latest version again.