Set the services to be included as an option in the module.

flake.nix

@@ -6,26 +6,24 @@
   };
 
   outputs = { self, nixpkgs, flake-utils }: {
-    serviceConfig = self.lib.makeServiceConfig
+    serviceConfig = self.lib.makeServiceConfig {
-      [
+      module = { imports = [ ./modules ]; };
-        "service-1"
+    };
-        "service-2"
-      ];
 
     lib = {
-      makeServiceConfig = serviceNames:
+      makeServiceConfig = { module }:
         let
           system = flake-utils.lib.system.x86_64-linux;
           lib = nixpkgs.lib;
           nixosConfig = nixpkgs.lib.nixosSystem {
             inherit system;
             specialArgs = { };
-            modules = [ ./modules ];
+            modules = [ module ];
           };
           services = lib.flip lib.genAttrs
             (serviceName:
               nixosConfig.config.systemd.units."${serviceName}.service".unit)
-            serviceNames;
+            nixosConfig.config.service-manager.services;
         in
         nixpkgs.legacyPackages.${system}.writeTextFile {
           name = "services";

modules/default.nix

@@ -1,10 +1,10 @@
-{ pkgs, ... }: {
+{ lib, pkgs, ... }:
-  systemd.services =
+
-    let
+let
   service-1 = "service-1";
   service-2 = "service-2";
-    in
+
-    {
+  services = {
     ${service-1} = {
       enable = true;
       description = service-1;
@@ -42,4 +42,16 @@
       '';
     };
   };
+in
+{
+  options = {
+    service-manager.services = lib.mkOption {
+      type = with lib.types; listOf str;
+    };
+  };
+
+  config = {
+    service-manager.services = lib.attrNames services;
+    systemd = { inherit services; };
+  };
 }