for a file in the cloud, cannot append to it.
this has been discussed in the forum
https://forum.rclone.org/t/does-rclone-works-with-rsync-to-upload-just-the-delta-changes-instead-of-complete-file-change/13738
might want to use a block-based backup.
rclone supports restic backup program
https://rclone.org/commands/rclone_serve_restic/