Turns out I took the wrong memory chip out. Now I have a combination of chips (including some older, smaller, ones) that earned me a "PASS" banner.
I am still going to rebuild the file system, I'm afraid there are deeper corruptions, but that will have to wait a little while. I've bought a new disk to replace the one that has been constantly-on since 2016 (the other one in the RAID is from 2019, so practically still brand-new).