Contao 5 Page Controller with Layout. Use Custom Controller and Templates within the contao layout. Build with reusable trait. Example replace the main section of the template. You can also replace another section and activate the content composition and add article to other template sections.

CustomPageController.php

<?php

namespace App\Controller\Page;

use Contao\CoreBundle\Controller\AbstractController;
use Contao\CoreBundle\DependencyInjection\Attribute\AsPage;
use Contao\PageModel;
use Symfony\Component\HttpFoundation\Request;

#[AsPage(type: 'custom_page',path: '/custom', contentComposition: false,)]
class RegistrationStatusPageController extends AbstractController
{
    use CustomPageTrait;

    public function __invoke(PageModel $pageModel, Request $request): Response
    {
        return $this->executeRender(
            $pageModel, 
            ['request' => $request,] // pass arguments as needed, for example passed url parameters
        ); 
    }

    function getContent(PageModel $pageModel, array $arguments): string
    {
        // do some fancy stuff
      
        return $this->twig->render(
          '@Contao_App/controller/custom.html.twig',
          $context,
          
        );
    }
}

CustomPageTrait.php

<?php

namespace App\Controller\Page;

use Contao\FrontendIndex;
use Contao\LayoutModel;
use Contao\PageModel;
use Contao\PageRegular;
use Symfony\Component\HttpFoundation\Response;

trait CustomPageTrait
{
    private array $arguments;

    public function executeRender(PageModel $pageModel, array $arguments = []): Response
    {
        $this->arguments = $arguments;
        $GLOBALS['TL_HOOKS']['generatePage']['custom_page'] = [self::class, 'renderPageContent'];
        $response = (new FrontendIndex())->renderPage($pageModel);
        unset($GLOBALS['TL_HOOKS']['generatePage']['custom_page']);

        return $response;
    }

    public function renderPageContent(PageModel $pageModel, LayoutModel $layout, PageRegular $pageRegular): void
    {
        // replace the main section of the template. Of course you replace another section
        $pageRegular->Template->main = $this->getContent($pageModel, $this->arguments);
    }

    abstract function getContent(PageModel $pageModel, array $arguments): string;
}