This

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

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.