Thanks for asking the question, it's right, there are thousands of backup programs already.

The "key thing" that lead me to code my own is that with nearly all the solutions I tried, data was resent over network when a big file is moved to another dir+renamed, WHEN used in encrypted-at-rest mode.

It's the case for rsync (the --fuzzy only helps when renamed but stays in same dir), duplicity, and even rclone when in encrypted mode (see https://forum.rclone.org/t/can-not-use-track-renames-when-sy...: "Can not use –track-renames when syncing to and from Crypt remotes").

So I wanted to solve this problem, and it works. See point #3 of https://github.com/josephernest/nfreezer#features.