I consider client devices to be a big risk factor and if I can keep them from having direct access to the Backup NAS and the IoT I consider that a big win. A simple ransomware attack on a client device would find any NFS/SMB shares the client can access and start encrypting - having the Backup NAS on a separate VLAN that only the server can access stops most of those from affecting the backup and makes restoring a lot easier. I would definitely recommend having an offline backup of the NAS as well in case of the server being breached.