> run ZFS scrub (or equivalent) every 6 months

zfs in mirror mode offers redundancy at the block level but scrub requires plugging the device

> All error correction has a limit. If too many errors build up, it becomes unrecoverable errors

There are software solutions. You can specify the redundancy you want.

For long term storage, if using a single media that you can't plug and scrub, I recommend par2 (https://en.wikipedia.org/wiki/Parchive?useskin=vector) over NTFS: there are many NTFS file recovery tools, and it shouldn't be too hard to roll your own solution to use the redundancy when a given sector can't be read