Update to add: The crashes kept coming, and as it almost always is in this case, it's something hardware related. In this case the culprit is the RAM (pretty sure), just changed it to Kingston HyperX Fury/Renegade 3600, also 128GB. Since the change required some disassembly I also changed the CPU to Ryzen 9-3950X. What's not to like Since this thing is running VMs and containers, more cores are a good thing.
Why am I so sure regarding RAM: I had similar issues with this kit in completely different hardware, after 18 months. So there appears to be a degradation issue. I changed everything (!) else in the box, same behaviour. Changed the RAM, stable... although the MSI boards seem to have ageing effects, too.
I will close this issue now, it is at least linked to the hardware issue. There was no useful info in the rsyslog, btw. last entry was some cron hourly job, then completely unresponsive box.
rookie701010's post in Crash during rebuild, changed one disk from 4TB to 8TB was marked as the answer