This sounds super interesting! Do you have an example of such a config somewhere, that you can share?
As far as the nixos config is concerned, there is nothing crazy in it. It is just a regular nix config with nginx (https://nixos.wiki/wiki/Nginx). You can see there that adding nginx is just four lines of code, ~eight with ssl. Use it to proxy to your applications that are setup as systemd services (https://wiki.nixos.org/wiki/Systemd/User_Services).
Edit: btw I'm a big fan of asciinema! ty for making it. :)