How do I display only nested form elements in modal

Question

i'm currently working on a custom field/widget.

For a better ux i want to display some nested elements in a modal dialog. The elements are generated during form submission (multistep widget) i added a custom ajax callback like this: (For better understanding i reduced the code)

public static function myAjaxCallback(array $form, FormStateInterface $form_state) {
    $response = new AjaxResponse();

    $form['#attached']['library'][] = 'core/drupal.dialog.ajax';
    $response->setAttachments($form['#attached']);

      $button = $form_state->getTriggeringElement();
      $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -3));

      $response->addCommand(
        new OpenModalDialogCommand('Only a few sub elements', $element, [])
      );

      return $response;
} 

The problem in this function is the elements are, obviously, rendered without a form around and so they can't get submitted.

Which would be the correct way to display only some nested elements in an ajax dialog?

Best regards Michael