works like this

modules/niri/default.nix

@@ -20,6 +20,7 @@
       home.packages = with pkgs; [
         brightnessctl
         swaybg
+        swayidle
       ];
 
       programs.niri = {
@@ -43,16 +44,6 @@
         settings = import ./swaylock.conf.nix { lib = lib; config = config; };
       };
 
-      services.swayidle = {
-        enable = true;
-        events = [
-          { event = "before-sleep"; command = "swaylock"; }
-        ];
-        timeouts = [
-          { timeout = 5; command = "swaylock"; }
-        ];
-      };
-
       services.mako = {
         enable = true;
         defaultTimeout = 5000;

modules/niri/niri.conf.nix

@@ -79,7 +79,7 @@
     { command = ["wpctl" "set-volume" "@DEFAULT_AUDIO_SINK@" "0"]; }
     { command = ["brightnessctl" "s" "50%"]; }
     { command = ["swaybg" "-c" "1f1d2e"]; }
-    { command = ["systemctl" "--user" "restart" "swayidle.service"]; }
+    { command = ["swayidle" "-w" "before-sleep" "swaylock"]; }
   ];
 
   binds = {