This acheives full file system backups to a destination. I this for backing up my entire Raspberry Pi SD cards (with a few minor modifications) daily via my crontab.
The rrsync script is required from https://www.samba.org/ftp/unpacked/rsync/support/rrsync
Sample line from authorized_key file must reside on the destination server where the backups will be. The path to backup it given here and all restrictions are in place so that this is the only thing the ssh user can do, and the only place they have access to. A public/private key combo without passphrase must have been created and shared out first etc. etc. https://www.guyrutenberg.com/2014/01/14/restricting-ssh-access-to-rsync/