Raspberry Pi, USB HD, with Tailscale and rsync?
Data Hoarder
We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
Add cron and I think you have a solution.
I used an old NUC with ubuntu to do this, except the backup was performed by my server rather than software on the NUC, makes it with easier that way, imo. Auto start in bios and Wireshark set to always connect.
I do recommend a wired connected if possible. So if someone messes with the wifi it won't matter.
What kind of nas do you have? Usually they have a good way to backup between their machines. I'd go with a lower spec version of what you've got, provided youre happy with it.
Actually that would be ONE (probably the only) use for the MyCloud things where you can enable ssh, and it comes with a usable ssh with sftp subsystem (can be used as rclone backend) and rsync binary (you can do rsync over ssh). Not that is for the MyCloud without "Home", which are much more "cloudy" things.
Otherwise of course anything else that has USB (and the network you want, wired or wifi) and runs ssh/rsync would do, including Windows if needed but most likely a Raspberry Pi or something.
How much data?