RAID-1, RAID-10, RAID-5, RAID-6, etc. none offer protection against an accidental `rm -rf`.

With that in mind, the NAS is not the backup. Once a file is deleted, it's gone.

With regard to recovering from a RAID-5 dual disk failure, typically a USB 2x sata toaster is enough to connect the failed disk and the replacement disk to a system that can run a ddrescue to clone the failed disk. Often times disks that are ejected from a RAID array still have enough life left to copy data off very slowly (over several tries/passes)

For backup, a simple solution is connecting very large external USB3 disk(s) and running daily rsync hardlink backups. There are several scripts to do this, and it's quite low effort to maintain after its set up.