You could set it to use your own DNS server, and have the server block anything not on a whitelist.

So… a business card?

Another advantage of Nextcloud over Syncthing is selective syncing: Syncthing replicates the entire collection of synced files on each peer, but Nextcloud lets clients sync and unsync subfolders as needed while keeping all the files on the server. That could be useful for OP if they have a terabyte of files to sync but don’t have that much drive space to spare on every client.

Have the new rail lines reduced automobile traffic? Or are they adding lines in anticipation of future traffic?

A typical use case is to forward a single port to the proxy, then set the proxy to map different subdomains to different machines/ports on your internal network. Anything not explicitly mapped by the reverse proxy isn’t visible externally.

Also Doctorow’s novella “Unauthorized Bread”.