You can use a Software like Veeam Agent for Linux or Acronis (not the one you get as a Plesk extension, as this one only supports backups into the Acronis cloud) to perform a full server backup and store it on a remote network device like your NAS.
We often use Veeam for that and the free version will allow you to connect to a SMB or NFS target as backup destination. (should be supported by your NAS)
The only problem I see here, is that these connections may not be encrypted, well maybe except if you can use SMB3
The paid version of said software does also support object storages (s3 compatible), but I don't know of your NAS does. (out of the box of with an extension like MinIO)