I do not use Google drive. I would rather prefer my nextcloud also for syncing settings between devices. Is there a backup file to transfer for this? What is the name and directory of such file?
You are right, either an rsync server or better an ssh server to rsync encrypted through an ssh tunnel. Syncopoli has an internal rsync client thst supports rsync and rsync over ssh.
Another option is RCX. It is an Android version of rclone which can copy to all kind of systems: SMB, rsync/ssh, webdav, gdrive, onedrive, dropbox, etc. It is not so "automatic" though and also the latest version has issues on Android 11 to access /android/data (same as syncopoli and many other apps currently on Android 11). Since a few days I am on Anroid 11.
So since a few days I use again TotalCommander with the Drive plugin for Google Drive, and the LAN plugin for SMB/Samba/Windows, and the WebDAV plugin.
Such a great tool. It only requires manual user actions.
Edit: Actually Ghost commander is likewise, completely open-source and even has slightly more functionality.