Alter Address field in order to add a list of cities

Question

I need to alter the address field because I want to transform the locality textfield in a nested select box along with administrative_area value.

I've tried to do that with a hook widget_alter, form alter, after_build callback, but any of them run my ajax callback.

Here is what I'm trying to do:

function ffcore_field_widget_address_default_form_alter(&$element, FormStateInterface $form_state, $context) {
  $element['address']['#after_build'][] = 'ffcore_validate_cities';
}

function ffcore_validate_cities(array $element, FormStateInterface $form_state) {

  foreach (Element::getVisibleChildren($element) as $key) {
    if (!empty($element[$key]['#required'])) {
      $element[$key]['#required'] = FALSE;
    }
  }
  $element['locality']['#weight'] = 1;
  $element['locality']['#prefix'] = '<div id="change-city-list">';
  $element['locality']['#suffix'] = '</div>';
  $element['administrative_area']['#weight'] = 0;
  $element['administrative_area']['#ajax'] = [
    'callback' => 'ffcore_change_cities_by_dept',
    'event' => 'change',
    'wrapper' => 'change-city-list',
  ];
  return $element;
}

function ffcore_change_cities_by_dept(array $form, FormStateInterface $form_state) {
  return $form['locality'];
}

In that way, I can change the order of the fields, but I can't use the ajax callback.

Is there any other way to achieve that?