You can push the rsync output to your NAS easily. I am using at home the WD My Cloud Ex2 NAS and the SSH is listening on a different port than the 22. So AFTER opening the 2233 port at the firewall of my router, I'm doing this:
rsync -a -e "ssh -p 2233" /home/user-data/backups/encrypted/ firstname.lastname@example.org:/path/where/to/backup
Don't forget the encryption key!
You may need to delete the local backup though after the transfer, but this might be a good example for a different cronjob entry.