In reply to@mib:kanp.ai

if you wanna go about it like this, you could e.g. have a bunch of modules (like you do), but define a top-level configuration option for all your stuff, i.e.

nixosConfigratuinon = {
  langhus = lib.nixosSystem {
    modules = [
      ./hosts/langhus
      /* the rest of your modules */
    ];
  };
};

then ./hosts/langhus/default.nix:

{ ... }: {
  options.custom = {
    hostname = "langhus"; /* although tbh you should just set `networking.hostName` and if you need to refer to it elsewhere use `config.networking.hostName` */
    /* etc */
  };
}

then your networking is imported just like all the other ones, but defined as

{ config, ... }: {
  networking.networkmanager.enable = true;
  networking.hostName = config.custom.hostname;
}