Load a page with only the form, no drupal theme/element around
I know it's possible to have a Controller action return raw HTML, by returning a Symfony\Component\HttpFoundation\Response object with the html string as an argument. IE

namespace Drupal\my_module\MyController;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\Response;

class MyController extends ControllerBase {
  public function content() {
    $markup = '&lt;h1&gt;Behold my markup!&lt;/h1&gt;';
    return new Response($markup);
  }
}


If that's what you are going for, you could hard code the form instead of using the drupal form api. It does mean abandoning the functionality of the form api as well though - you are basically creating a route to a plain html page.