I wrote a how-to!

https://troyvit.net/2023/01/backing-up-android-to-rsync-net/

It has one error. termux-job-scheduler isn't a separate install.

One extra tip is to use rrsync to restrict which folders your rsync client can read/write to: https://b.tuxes.uk/termux-rsync-photos.html

This helps mitigate ransomware, and escalation (e.g. reading previously backed up files) attacks.