Way to Defer Laravel's API Request Validation?

I have an API resource that accepts a request as such:

 public function create(StorePostRequest $request) {
 }

The problem is, when I send a POST request to this create() endpoint, the rules() defined in StorePostRequest.php kick in before the method is even called. I want to do some pre-validation and THEN validate with the rules() in the StorePostRequest.

The rules() that are defined in StorePostRequest.php kick in before the create() method is even called.

    public function rules()
    {
        return [
            'title'                  => [
                'min:10',
                'max:255',
                'required',
            ],
            'slug'                   => [
                'min:10',
                'max:255',
                'required',
                // 'unique:posts',
            ],
            'user_id'                => [
                'required',
                'integer',
            ],
        ];

For example, the user_id should be retrieved from the Auth() session, and not get passed in. But I do want the rule to be there when the data gets stored to the posts table.

So, I changed the API to:

    // Note: if you accept a StorePostRequest $request, it automatically pre-validates based on rules() in StorePostRequest
    public function create(Request $request)
    {
        $loggedinUser = auth()->user();
        if (!$loggedinUser) {
            return response(["error" => "User is not logged in. User needs to be logged in to use this API."], Response::HTTP_FORBIDDEN);
        }
        $loggedinUserId = $loggedinUser->id;

        $requestData = $request->all();
        if (!isset($requestData['title'])) {
            return response(["error" => "title is required"], Response::HTTP_BAD_REQUEST);
        }
        if (!isset($requestData['body'])) {
            return response(["error" => "body is required"], Response::HTTP_BAD_REQUEST);
        }
        if (!isset($requestData['status'])) {
            return response(["error" => "status is required"], Response::HTTP_BAD_REQUEST);
        }
        $requestData['slug'] = $this->createUniquePostSlug($request->title);
        $requestData['user_id'] = $loggedinUserId;
        $requestData['description'] = $request->title;
        $requestData['reading_time_minutes'] = ceil(str_word_count(strip_tags($requestData['body'])) / 200); // assuming an average reading speed of 200 wpm

        $post = Post::create($requestData);

        return (new PostResource($post))
            ->response()
            ->setStatusCode(Response::HTTP_CREATED);
    }

However now I have to do a bunch of manual validation (using isset($requestData) like I'm doing above. I want the StorePostRequest rules to kick in, but after I do some request validation myself. Is there a way to do this? Maybe there's a way to cast the Request object to the StorePostRequest? THoughts? And thanks in advance.