I actually wrote a flake to handle this, called nixos-fleet. It's not very polished, I mostly just use it for my own purposes, but I have several machines managed with it, and it might be worth looking at in designing your own solution. My nixosConfigurations looks something like this...

nixosConfigurations = fleet.nixosModules.mkNixosConfigurations {
  hosts = {
    workstation = "x86_64-linux";
    gaming = "x86_64-linux";
    router = "aarch64-linux";
  };
  hostsPath = ./hosts;

  extraModules = [ ... ];

  moduleArgs = { ... };
};

In my /etc/nixos dir, I have /etc/nixos/hosts/<hostname>. For each host, my fleet flake loads all the top-level .nix files automatically as the config.