While NAS has mirror on it, I do not consider it enough backup. I assume it will not save me from ransomware or a family member stupidly deleting a lot of data or from fire or from theft. It is mostly to give convenient access to it for my family.
HDDs are in rotation and one is always offsite with my family that I visit regularly. Whenever I travel there I make a backup on one of the HDDs and then bring the "old" drive back home to be reused on one of future trips.