Although I might be talking to myself here is another update:
My system is running stable now for 9 days without any freeze.
The changes I did:
remove the following boot parameters:
amd_iommu=oniommu=pt
Reason I had set them were severe problems with horizontal glitches/flickering lines six month+ ago.
These problems went away after setting the boot parameters so I left them in until now. As the problems did not appear again after removing, the underlying problem seems to be solved.
I added the following boot parameter:
amdgpu.noretry=0
Source: https://www.phoronix.com/scan.php?page=news_item&px=AMDGPU-APU-noretry
In addition I reset BIOS to default settings and explicitly disabled IOMMU afterwards (switch from AUTO to DISABLED).
Not sure if the BIOS reset is required.
Fingers crossed that the system remains stable.