How big is your existing RAID 1 NAS? (The one you make your backups to.)
The reason why I ask is this: Let's say you have a 2 TB NAS that you make your backups to. So that means you can backup a maximum of 2 TB total (and that doesn't allow any versioning / differential backups).
So if your new NAS could store, say, 4TB in RAID 0, then anything that you put on it over 2 TB can't be backed up to the current backup system. Make sense?
You will get additional storage space, and possibly increased write speeds (depends on the interface, drives, and controller) with RAID 0. RAID 1 will protect you from a single hard drive failure, that's it. It will also give you half the storage space of the RAID 0 option. JBOD, in my opinion, is only useful with very specialized situations such as creating a Linux zfs filesystem, which automatically takes care of providing redundancy and volume enlargement capabilities over multiple drives. (Creating a Linux zfs filesystem is a rather advanced technique, by the way, so it's not for the computer beginner.) Otherwise, you're better off letting your NAS decide how to spread the data over the available drives.
Ultimately, I think your decision is going to be based most on realistically determining how much data you currently have and how much data you anticipate having in the next 5 or so years (and your present ability to back it up). And your choice will be essentially between RAID 0 and RAID 1.
Sorry I provided more questions than answers, but those are some of the variables involved in that decision.