Pimcore/Symfony Bundle with custom token authentication for custom controller

I am completely struggling to get a custom bundle in Pimcore working as expected. I have a controller in this bundle that must be secured by a token authentication (like a Bearer token). This is my security.yml in BUNDLE_DIR/Resources/config/security.yml:

security:
providers:
    noop_user_provider:
        id: MyVendorPimcoreApiBundleSecurityNoopUserProvider

firewalls:
    api:
        pattern:   ^/api
        stateless: true
        guard:
            authenticators:
                - MyVendorPimcoreApiBundleSecurityTokenAuthenticator

access_control:
    - { path: ^/api, roles: IS_AUTHENTICATED_FULLY }

My controller route is as follows (via annotations):

/**
 * @param string $id
 * @return JsonResponse
 * @Route("/api/read/{id}", name="myvendor_pimcore_api_read", methods={"GET"})
 */
public function read(string $id): JsonResponse
{
    return $this->json([
        'message' => 'Hello from your custom API! You ID was: '.$id,
    ]);
}

And my services.yml (directory: BUNDLE_DIR/Resources/config/services.yml) has the following entry:

    MyVendorPimcoreApiBundleSecurityTokenAuthenticator:
    arguments:
        $validToken: 'sampleToken123'
    tags:
        - { name: security.guard_authenticator }

And whatever I tried, I cannot get it to work. If I try to include my security.yml via DependencyInjection, I get an error:

public function load(array $configs, ContainerBuilder $container): void
{
    $configuration = new Configuration();
    $config = $this->processConfiguration($configuration, $configs);

    $loader = new LoaderYamlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
    $loader->load('services.yml');
    $loader->load('security.yaml');
}

I get this error: There is no extension able to load the configuration for “security” (in “/var/www/html/bundles/MyVendor/PimcoreApiBundle/DependencyInjection/../Resources/config/security.yaml”). Looked for namespace “security”, found “none”.

I also tried multiple other things, which all resulted in more or less the same issue. Can someone help me here? I just want to my controller to be secured…