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

@@ -1,45 +1,57 @@
-{ pkgs, ... }: {
+{ lib, pkgs, ... }:
-  systemd.services =
-    let
-      service-1 = "service-1";
-      service-2 = "service-2";
-    in
-    {
-      ${service-1} = {
-        enable = true;
-        description = service-1;
-        wants = [ "network-online.target" ];
-        after = [
-          "network-online.target"
-          "avahi-daemon.service"
-          "chrony.service"
-          "nss-lookup.target"
-          "tinc.service"
-          "pulseaudio.service"
-        ];
-        serviceConfig = {
-          Type = "oneshot";
-          RemainAfterExit = true;
-          script = ''
-            true
-          '';
-          ExecReload = "true";
-        };
-        wantedBy = [ "multi-user.target" ];
-      };
 
-      ${service-2} = {
+let
-        enable = true;
+  service-1 = "service-1";
-        description = service-2;
+  service-2 = "service-2";
-        serviceConfig = {
-          Type = "simple";
-        };
-        partOf = [ "${service-1}.service" ];
-        wantedBy = [ "${service-1}.service" ];
 
+  services = {
+    ${service-1} = {
+      enable = true;
+      description = service-1;
+      wants = [ "network-online.target" ];
+      after = [
+        "network-online.target"
+        "avahi-daemon.service"
+        "chrony.service"
+        "nss-lookup.target"
+        "tinc.service"
+        "pulseaudio.service"
+      ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
         script = ''
           true
         '';
+        ExecReload = "true";
       };
+      wantedBy = [ "multi-user.target" ];
     };
+
+    ${service-2} = {
+      enable = true;
+      description = service-2;
+      serviceConfig = {
+        Type = "simple";
+      };
+      partOf = [ "${service-1}.service" ];
+      wantedBy = [ "${service-1}.service" ];
+
+      script = ''
+        true
+      '';
+    };
+  };
+in
+{
+  options = {
+    service-manager.services = lib.mkOption {
+      type = with lib.types; listOf str;
+    };
+  };
+
+  config = {
+    service-manager.services = lib.attrNames services;
+    systemd = { inherit services; };
+  };
 }