Drupal 9, Webform 6 - custom element w/ ajax in form

AjaxInWebformElement.php

<?php

namespace Drupal\ocf_integration_webform\Plugin\WebformElement;

use Drupal\Core\Form\FormStateInterface;
use Drupal\webform\Element\WebformAjaxElementTrait;
use Drupal\webform\Plugin\WebformElementBase;
use Symfony\Component\HttpFoundation\Request;

/**
 * @WebformElement(
 *   id = "ajax_form_test",
 *   label = @Translation("Ajax form test"),
 *   description = @Translation("Provides a form element for a choosing an Open Collective event ticket."),
 *   category = @Translation("Options elements"),
 * )
 */
class AjaxInWebformElement extends WebformElementBase {

  use WebformAjaxElementTrait;

  protected function defineDefaultProperties() {
    return [
        'field_1' => '',
        'field_2' => [],
      ] + parent::defineDefaultProperties();
  }

  public function form(array $form, FormStateInterface $form_state) {
    $form = parent::form($form, $form_state);
    $form['field_1'] = [
      '#type' => 'select',
      '#title' => $this->t('Field 1'),
      "#validated" => TRUE,
      '#options' => [
        'one' => 'One',
        'two' => 'Two'
      ],
      // When the event_id changes, update the options field values.
      '#ajax' => [
        'callback' => '\Drupal\ocf_integration_webform\Plugin\WebformElement\AjaxInWebformElement::ajaxCheckboxes',
        'disable-refocus' => TRUE,
        'event' => 'change',
        'wrapper' => 'field_2_wrapper',
        'progress' => [
          'type' => 'throbber',
          'message' => t('Verifying entry...'),
        ],
      ],
    ];

    $form['field_2'] = [
      '#type' => 'checkboxes',
      '#title' => $this->t('Field 2'),
      '#options' => [
        'check_one' => 'Check One',
        'check_two' => 'Check Two',
      ],
      '#prefix' => '<div id="field_2_wrapper">',
      '#suffix' => '</div>',
    ];
    return $form;
  }

  /**
   * Ajax callback.
   */
  public static function ajaxCheckboxes(array &$form, FormStateInterface $form_state, Request $request) {
    $form['field_2'] = [
      '#type' => 'checkboxes',
      '#title' => t('Field 2 - Changed'),
      '#options' => [
        'check_three' => 'Check Three - changed',
        'check_found' => 'Check Four - changed',
      ],
      '#prefix' => '<div id="field_2_wrapper">',
      '#suffix' => '</div>',
    ];

    return $form['field_2'];
  }

}