OP! I had the exact same issue as you, it plagued me for 2 years but I just couldn't nail it down. I had a good quality Corsair PSU, but the kicker was that I had a spare Corsair PSU and I had the same issue. I tried so many things and diagnoses and sometimes it would seem to work, but return sporadically.

I eventually found out it was the VRMs on my motherboard! I can't explain WHY they became temperamental, my board was a B550i which isn't low-end, but was micro ATX so perhaps it was prolonged temperature..?

Anyway - I got a new motherboard and it 100% solved the issue. Same PSU. The old motherboard is now my NAS, it's stable enough without a GPU or display. If you have the opportunity to try another PSU, do it!