try

system-config/users/default.nix

@@ -4,55 +4,58 @@
         users = let 
 
             userType = types.submodule ({ username, ... }: {
-                options = {
-                    username = lib.mkOption {
+                options = with lib; {
+                    username = mkOption {
                         type = with types; passwdEntry str;
-                        default = username;
                     };
 
                     home-manager = {
-                        enable = lib.mkOption {
+                        enable = mkOption {
                             type = with types; bool;
                             default = false;
                         };
 
-                        standalone = lib.mkOption {
+                        standalone = mkOption {
                             type = with types; bool;
                             default = false;
                             description = "is this home-manager standalone?";
                         };
 
-                        extraModules = lib.mkOption {
+                        extraModules = mkOption {
                             type = with types; listOf raw;
                             default = [];
                         };
                     };
 
-                    sshKeys = lib.mkOption {
+                    sshKeys = mkOption {
                         type = with types; listOf str;
                         default = [];
                     };
 
-                    uid = lib.mkOption {
+                    uid = mkOption {
                         type = with types; nullOr int;
                         default = null;
                     };
 
-                    hashedPasswordFile = lib.mkOption {
+                    hashedPasswordFile = mkOption {
                         type = with types; nullOr str;
                         default = null;
                     };
 
-                    extraGroups = lib.mkOption {
+                    extraGroups = mkOption {
                         type = with types; listOf str;
                         default = [];
                     };
 
-                    shell = lib.mkOption {
+                    shell = mkOption {
                         type = with types; package;
                         default = pkgs.shadow;
                     };
                 };
+
+                config = {
+                    username = lib.mkDefault username;
+                };
             });
 
         in lib.mkOption {