Why doesn't form submit callback get called?

I have a custom page (controller) which embeds a node form. I want to submit this form via ajax but also run another submit handler which opens a modal with some of the values from that form.

My controller function is simple:

  public function createQuote() {
    $node = \Drupal\node\Entity\Node::create(['type' => 'mpep_quote']);
    $form = \Drupal::service('entity.form_builder')->getForm($node);
    $form['#attached']['library'][] = 'core/drupal.dialog.ajax';

    $form['open_modal'] = [
      '#type' => 'submit',
      '#value' => t('Open modal'),
    ];

    $form['actions']['#access'] = FALSE;

    return $form;
  }

This works fine to show the form and clicking my new submit button submits the form (non-ajax).

I have tried numerous (recommended and unsuccessful) methods of trying to get the form to submit using ajax, such as adding this to $form['open_modal']:

  '#ajax' => [
    'callback' =>  '::openQuoteModal',
    'event' => 'click',
  ],

where openQuoteModal is my additional submit function. This does not submit the form via ajax nor does it run my added submit handler.

The only way I have been able to get the form to submit via ajax is to add this to the $form['open-modal']:

'#attributes' => ['class' => ['use-ajax-submit']],

This does work to submit the form via ajax, but nothing I have done will run my added submit handler. I have tried the following:

1. #ajax callback as shown above
2. adding a #submit to the form submit button
3. adding a #submit to the form itself

Is there any way to get both the from to submit via ajax as well as run a secondary form submit handler?

Ajax forms need to have a wrapper with an id.

// Add wrapper for AJAX element.       
$form['#prefix'] = '<div id="ajaxfeedback">';       
$form['#suffix'] = '</div>'; 

And I think the callback needs to return an AJAX response.

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

    // If there are any form errors, re-display the form.
    if ($form_state->hasAnyErrors()) {
      $response->addCommand(new ReplaceCommand('#ajaxfeedback', $form));

      return $response;
    }

    $response->addCommand(new OpenModalDialogCommand("Success!", 'The modal form has been submitted.', ['width' => 500]));

    return $response;
  }

(reposted for formatting)