One of them appears to be broken [1]. No big deal, this is what RAIDs are for, I go and try to find one and now they're going anywhere between 2-4x that price, for a used one! It's not going to bankrupt me (and having a home server is a privilege in the first place, that's not lost on me), but I really hope that the others survive, at least until this storage crunch is over. If it ever does end...sigh.
I guess I didn't realize that even relatively slow storage like spinner drives was going to be affected too.
[1] I think, I am really hoping it's just a bad connection or something but I haven't fully diagnosed it yet.
ETA: Looks like at least in my case it was actually just a bad SATA cable. The drive is reading properly and resilvering now. Phew.