I've not seen many comments about Raid Z3 - how many physical disks are you running? What size are they?
Before commenting on backup strategies, it would be helpful if you gave an indication about how much data you are storing, and how often it changes. The inexpensive internet backup services are great when your data volumes are modest, but you might need a more expensive commercial service if you plan to back up large volumes.
I have two separate volumes: one for music, movies and photos, and another for backups from my PC's. I back up my PC's to the the NAS every day, and that volume is also backed up via an inexpensive internet service (all this is automated.) Once a month, I backup everything on the NAS to external disk and carry off-site. In this way, I create daily backups of my PC's - which are stored both locally and offsite - and a monthly backup which includes a snapshot of the PC data, plus all my media and photos. Since the media and photos don't change very often, I am comfortable with the notion of backing them up once a month.
Assuming a complete fail of my entire system, which is unlikely, the most data I have at risk will be any changes made to the media since the last external backup. If I need to restore files to one of my PC's, I can just copy the files from the local daily backup (or restore from the internet.)