Try a different tack...

objectifier-module.nix

@@ -57,15 +57,6 @@ in {
       after = [ "network-online.target" ];
       wantedBy = [ "multi-user.target" ];
       reloadIfChanged = true;
-      path = with pkgs.python310Packages; [
-        pkgs.python310
-        gunicorn
-        fastapi
-        gunicorn
-        opencv4
-        python-multipart
-        uvicorn
-      ];
       environment = {
         OBJECTIFIER_YOLOV3_CONFIG = "${pkgs.yolov3-data}/yolov3.cfg";
         OBJECTIFIER_YOLOV3_WEIGHTS = "${pkgs.yolov3-data}/yolov3.weights";
@@ -74,6 +65,7 @@ in {
         OBJECTIFIER_CLEANUP_MAX_AGE = toString cfg.cleanup.max_file_age;
         OBJECTIFIER_CLEANUP_DELAY = toString cfg.cleanup.delay;
       };
+      path = [ pkgs.objectifier ];
       serviceConfig = {
         # PrivateUsers = true;
         # PrivateDevices = true;
@@ -103,12 +95,10 @@ in {
             (map (addr: "--bind ${addr}:${toString cfg.port}")
               cfg.listen-addresses);
         in (concatStringsSep " " [
-          "gunicorn"
+          "objectifier"
           bindClause
           "--workers ${toString cfg.workers}"
-          "-k uvicorn.workers.UvicornWorker"
           "--pid /run/objectifier.pid"
-          "objectifier:app"
         ]);
       };
     };

objectifier.nix

@@ -1,12 +1,33 @@
 { pkgs, lib, ... }:
 
-pkgs.stdenv.mkDerivation {
+let
+  objectifier-src = pkgs.stdenv.mkDerivation {
+    name = "objectifier";
+    src = ./src;
+    phases = [ "installPhase" ];
+    installPhase = ''
+      mkdir -p $out
+      cp $src/detector.py $out/detector.py
+      cp $src/yolo-cli.py $out/objectifier.py
+    '';
+  };
+
+  pythonYolo = pkgs.python3.withPackages (pyPkgs:
+    with pyPkgs; [
+      gunicorn
+      fastapi
+      gunicorn
+      opencv4
+      python-multipart
+      uvicorn
+    ]);
+in pkgs.writeShellApplication {
   name = "objectifier";
-  src = ./src;
-  phases = [ "installPhase" ];
-  installPhase = ''
-    mkdir -p $out
-    cp $src/detector.py $out/detector.py
-    cp $src/yolo-cli.py $out/objectifier.py
-  '';
+  runtimeInputs = [ pythonYolo ];
+  text = pkgs.lib.concatStringsSep " " [
+    "gunicorn"
+    "objectifier:app"
+    "-k uvicorn.workers.UvicornWorker"
+    ''"$@"''
+  ];
 }