[NV+ v2] 4 disks, 2 separate RAID-1 volumes?

You are falling into the trap of using RAID as a backup. The manuals and everyone with experience states flatly that RAID is NOT a backup solution. Separate backups must be maintained to protect against the failure when RAID will not save you. If you only lose one drive in an array and have no other problems, then that is the protection that RAID was developed for. However, in too many cases, the failure is beyond the salvage by RAID. These examples are a failed upgrade, a software glitch during some operation that creates the dreaded kernel panic error, fire, theft, hardware failure of the NAS itself. The pages of these forums are full of posts by members whose units have a problem that RAID was unable to resolve.

I ran for years with a two drive array holding all my critical and important data with only a sporadic backup. Fortunately, the only problem I had during this time was the simple failure of one drive which was easily resolved by replacing the bad drive. However, I would be remiss in recommending this as an operating plan.