Freenas Internal disk backup

I have some critical files in my main dataset that I would like to have mirrored to a backup dataset as a backup.
I am backing up the entire setup on monthly basis to offsite server with rsync, but I would like to have some additional peace of mind.

Will a simple rsync script do? If so, what would be best way to make this happen automatically? Even better would be some kind of incremental backup like time machine.

Why not use deja-dub on your machine to an internal hdd? Automatic incremental backup.