Good idea to use a scripted RCU to do a backup. Not sure its CLI allows it, though? You can do a backup from the UI by selecting all (ctrl-A) and click the download button.

You could also do a full backup with rsync, basically just

rsync -rtb --delete -v rm:/home/root/.local/share/remarkable/xochitl

I used this in a script, which I start whenever I made a significant change on the rM: https://github.com/RedTartan04/rMbackup

My backup directory is also a local git repo, so I can keep versions.

Use the search function for other backup solutions.