Syncthing is good for copying files around. For backup, a proper backup tool is required. I've used Titanium Backup for 14 years (eek!), recently switched to Swift Backup, and am now switching to O&O/NeoBackup.
There's also Seedvault, which I haven't tested yet.
With Android, I do local backups and let Syncthing transfer the backups to my server when I'm on WiFi and charging.