Just tried sorting out a background backup solution using Deja Dup on Ubuntu. It turns out to be a bit less intuitive than I had hoped, but it looks like it's working now. This is how I set it up:
1) Deja Dup config screen has 5 sections: Overview; Folders to save; Folders to ignore; Storage location; and Scheduling. We can ignore Overview - it's self-explanatory.
2) Folders to Save can contain a number of absolute paths to local directories you want to back up.
3) Folders to ignore. Literally that.
4) The beef. This is where it's not intuitive. If you have remote storage that you want to access using sftp/ssh you need to use the following syntax:
Choose 'Network Server' from the list
In 'Network location' use 'ssh://machine.local' - this is for a remote machine on your local network.
In 'Folder', use the aliased form of the directory to store your stuff in. In the case of a Synology NAS DO NOT use the absolute path to your home directory (starts with var) use the aliased version (starts with homes)
5) Scheduling. As you see fit.