Syncthing is excellent for phone sync.
What I did was have it running on a system in the network of the nas, mount the nas on that system, and place the backups folder in the nas.
If you have a system that reliably runs, or can get syncthing running on the nas, I recommend doing that.
Synology has docker iirc, there aught to be a syncthing container.
Else, slapping a pi zero into the nas' network should do the trick and be fully independent of what the nas is.