I only see 2 (affordable) options:
- a 2nd NAS (at home or at another location)
- Using online backup/storage with something like backblaze or crashplan. (how are you going to manage encryption?)
Or would an USB external drive be sufficient? However, you will take some, albeit low, risk with a single disk. The chance both your server and the usbstorage dies together is very low. It might happen, for example when your house gets destroyed, but then I think you have other things to worry about when that happens.