The server is configured as a caching reverse proxy (just nginx with cache.nixos.org as upstream) which I think is similar to squid proxy.

Outside my LAN I do have the ability to tunnel home, but depending on the connection and the updates I just deal with the timeouts, or just wait until I get home.

I think technically you can override substituters using cli options but its not ideal. There are several proposals for configuring timeouts per substituter but none of them are merged yet.