I stumbled upon cyanrip, which seemed to be the most complete and up-to-date cli utility, and it did not disappoint! Highly recommend!

I did, yes, but no avail. The dmesg output I posted is after the drive was mounted as ro, and is the best i could get. After some time, the system stops responding completely

Unfortunately I don't have a spare PSU, but I might try to measure the 3.3 volt rail with a multimeter (don't own an oscilloscope unfortunately) while under load and see what happens

Thanks, I'll try that. I loaded the drive using dd a couple of times, and that did bring the system down a couple of times. I was writing to the filesystem though, while the system was booted

Unfortunately I have no other system at hand at the moment that's able to accept nvme drives :( I could try using windows for a couple of days see whether the issue is really linux-related, but I am trying to avoid that lol

Boot a live ISO with the flags recommended in the kernel message and do some tests on the bare drives. That way you won’t have the filesystem and subsequently the rest of the system giving out on you while you’re debugging.

Which tests are you referring to exactly? I have read about badblocks for example, and it not being much use for ssds in general, due to their automatic bad-block-remapping, so they remain invisible to the OS as all remapping happens in the drive's controller. Smart values look great for both drives, about 20TBW on the Samsung drive, and a lot less on the Kioxia drive.

Happens with both drives, I have tried each possible permutation (Samsung in slot 1 and 2, kioxia in slot 1 and 2, and even only installing one drive at a time)

I did, yes. One of the first ideas I got.