Those same questions always bug me, and I did try all from very smart to very brute force solutions. I love ZFS but then we can question ZFS and OS bugs in the same manner as saf or rsync -- that rabbit hole is deep and quickly becomes expensive since ZFS may need ECC ram and other more expensive components.

Lately, in last few years, I am leaning towards using many cheap backups instead of clever and more expensive ones, with the idea that many of them can't all break at the same time. Yes occasional checks are good but safety in numbers seems as a good strategy.

It is not an accident that saf tag line says "one backup is saf, two are safe, three are safer" ;)