Incognito Clown

Thanks @paddy-stone and @Valkyrie Lenneth. It turned out to be my PSU after all.
I switched the power supply in my gaming rig (the faulty one) with the one from my NAS. Both systems ran fine after that for 1,5 months. When swapping them back my gaming pc crashed again within 2 days. At that point I was pretty sure the issue was with the PSU. The NAS is on 24/7 but uses way less power than my pc (<50w I'd guess). Seems that the PSU was just no longer able to reliably handle higher loads.
 
I sent the PSU in for warranty to Corsair. They sent me a new one within a week. Great service ?
The new one has been working flawlessly for a month now ☺️

In case anyone sees this from a search - the 3 frame-to-motherboard screws were too tight.
 
After re-watching der8auer's video about it, I noticed that he did not tighten them very much (he stops when they are tight enough to move the free-standing motherboard). After seeing that I pulled my water block, cracked all 3 loose and re-tightened them using only my index finger and thumb to gently twist the short end of the included star wrench just until there was resistance.
 
Remounted the block (thank you liquid metal for not having to redo TIM), fired her up, changed RAM speed in BIOS back up to 3200, save and reboot. Voila, problem solved.

It still retails for €300 new, though there might be some used available for less. However, I do prefer to just go with a new platform. This does mean I have to upgrade my motherboard as well, but since my current motherboard was really on the budget side of things (less than 100 euros/dollars), thats not a big deal.
 
Thanks for the suggestion though!
 

nothing fancy just simple cuting and some paint