It's definitely an option. Though I'd really need 2 drives - one replacement, and a mirror. And then I could set up weekly sweeps which check zfs checksums to guard against bit rot. And just hope that bitrot never affects the same file on both drives at the same time.

I also don't want all my backups to be in the same physical location. If I copy some files with zfs to another drive, I'd also like to check that there wasn't an error in the copying process. Maybe I can use zfs to make mirror images - which probably copies the hashes?

Yeah, I could use zfs to do some of this. But having a sidecar checksum database is easier to work with across filesystems. And Reed-Solomon error correction is able to correct bit rot much more cheaply than using physically duplicated drives. I wouldn't mind buying a mirror set a year or two ago, but storage is so expensive now.

Filesystems in general aren't very good at protecting against silent bitrot, since it doesn't happen that much on spinning copper. But it happens a lot more than we'd all like to admit on modern SSDs. And zfs doesn't support Reed-Solomon error correction.