There’s different kinds of backups. For this you don’t need off-site storage.
For this I set up zfs auto snapshotting which means when I delete stuff it isn’t really deleted because a snapshot is still pointing at it until it rolls off the time window.
Both zfs and btrfs can do this but you do need to change the filesystem to use these which can be a lot of work.
Black holes merge. It doesn’t matter what size they are. It’s not that the bigger one eats the smaller one they just merge.