Close webform modal and redirect to entity
Yes you are so close!
just move the creation of the new entity from your postSave()
to right in the ajax call back

example:

_CUSTOM_MODULE_webform_submission_form_submit_ajax(array &amp;$form, FormStateInterface $form_state) {
 // At this point we have all access to $form_state-&gt;getValues();
 // the webform submission is all ready saved so we can use the values here.

 // Get the entity storage.
 $entity_storage = \Drupal::entityTypeManager()-&gt;getStoage('ENITYNAME');

  // Create New entity.
  $new_entity = $entity_storage-&gt;create([
   'field_whatever' =&gt; $form_state-&gt;getValue('some_field');
  ]);
  // Save entity
  $new_entity-&gt;save();
  $new_entity_id = $new_entity-&gt;id();
  // set the url. 
  $url = "/where-ever/" . $new_entity_id;

  $response = new AjaxResponse();
  $response-&gt;addCommand(new CloseModalDialogCommand());
  $response-&gt;addCommand(new RedirectCommand($url);
  // Party.
 return $response;
}