I just found out about autorandr and it completely made my day: you can save screen profiles that are automatically loaded when there is a screen connected/disconnected (and it can even call scripts on changes).
Quite obviously, on NixOS, it's just one line of config to enable it:
services.autorandr.enable = true;
and "nixos-rebuild switch" away!
Linux Geeks doing what Linux Geeks do..