For DIY consider a setup that supports ECC RAM to help prevent corruption. Any good server motherboard should do.

Unraid is pretty easy to get going on. That’s probably the direction I would take in your situation.

Also, if you’re not doing 3-2-1 backup now might be a good time to consider an off-site backup plan. That 4-bay Synology at a friend’s house with a VPN path would be an option for critical data. You could give them some partitioned space on there and on your new NAS to compensate for the power usage. Setup Borg or Restic and it’ll be encrypted on the remote NAS, and benefit from incremental and dedupe to minimize bandwidth usage.

It’s Munster on the list, a washed-rind cheese from France. Muenster cheese in America is probably somewhere down around a used Ford Focus.

How about creating your own LAN within the untrusted network?

Something like an inexpensive OpenWRT router would do fine. Connect all your devices and the server to the router. They are now on a trusted network. Set up Wireguard on the OpenWRT router to connect to Proton so that your outbound traffic from all your devices is secured.