I see syncthing being recommended, and like, it's fine.
But keep in mind it's NOT a backup tool, it's a syncing tool.
If something happens to the data on your client, for example, it will happily sync and overwrite your Linux box's copy with junk, or if you delete something, it'll vanish in both places.
It's not a replacement for recoverable-in-a-disaster backups, and you should make sure you've got a copy somewhere that isn't subject to the client nuking it if something goes wrong.