The rule is simple: three copies, two kinds of media, one copy somewhere else. The hard part is not the rule, it is making sure the job still runs six months later when nobody is watching.
Photos and documents live on the main machine. Every night a restic job pushes a snapshot to a USB disk, and once a week the same repository is synced to a second disk that stays at a relative's place.
restic -r /mnt/usb/restic backup ~/Documents ~/Pictures --exclude-caches
restic -r /mnt/usb/restic forget --keep-daily 7 --keep-weekly 8 --keep-monthly 12 --prunerestic check --read-data-subset=5% to catch silent corruption.