Nixos is perfectly usable without an Internet connection. I've never encountered an issue, and in fact I've joked with my wife that considered as an overall end-to-end system (i.e. including the Internet dependency), my jellyfin instance gets better uptime than something like Spotify would.

You can't install or update new software that you'd pull from the web, but you couldn't do that with any other system either. I can't remember specifically trying but surely if you're just e.g. modifying your nginx config, a rebuild will work offline?