Thanks for the suggestion! I have just tried to stress-test it pretty intensively (without reseating), no issues found. Also, the freezes did not happen under GPU load but just randomly when browsing in Chrome or when I was away but Chrome was open in the foreground (both happen a lot, so I cannot exclude it could not freeze with other usage as well but definitely not associated with high GPU load).
I will try reseating it but since normally the freeze happens randonly once in roughly 1-2 weeks, it will take time to report back.
But, from your comment, I guess my issue could be more likely this one: Random Xid 61 and Xorg lock-up - #30 by collinvandyck
I also have a Ryzen CPU (2990WX) and an ASUS board (399A) which seems to be associated with that issue. That would be good news at least wrt the fix of the bug from this thread. So I’ll also try the suggestions from that thread…