• Move anonymous route functions from Routes.php and the RouteHandlers directory into controller classes, reference them in Routes.php (see AddDonation as an example).
• Remove all references to the Application class (for getting services like session, etc). Find a different way to inject the session into the controller, e.g. as a request attribute
• Replace calls to $app->json() with JsonResponse::create

One Controller class per route, except for routes that have POST and GET (then do two actions).