RAID is not backup. A NAS with a RAID may be an important part of an overall backup strategy, but if the RAID is not backed up do not rely upon it for backups.
RAID 6 may be tempting but consider RAID 5 and a solid backup strategy instead. It is faster and will rebuild quicker when you have a failure. When a RAID 6 fails there is A LOT of crunching that has to happen to rebuild and you flirt with a second and third failure putting the remaining disks to task like that, especially since you probably bought similar drives at the same time as the one that failed? Think about it and condider RAID 5.
Careful with hot swap. Consider not. After a failure and before a rebuild consider first verifying your backups; make sure they have been running properly. Check the new drive, use this as an opportunity to re-think the system, perhaps if you are doing SHR and your system has all 3TB drives, maybe you want the next drive to be a 5TB so you are one step ahead on expanding volume size? In any event if you choose Hot Swap make sure it is new and unused or if previously used, make sure that any residual RAID metadata on it has been deleted via the original RAID controller. Consider instead having a drive ready in a spare bay but stop just short of telling the system to immediately put it into use when a failure occurs.
A good backup system has versioning to protect against things like a file becoming corruputed but not immediately detected.
There is no substitute for off-site backups so get a good cloud service, another machine in another city or some recurring rotating backup system such as with LTO tapes.