How to refresh/rebuild block without reloading the page?

I have a custom block which displays the number of events added to favorites(Flag module). I am using the TWIG template to display those results. I want to refresh the block contents right after the user flags/unflags a node. Currently I can only get the updated value if I reload the page.

The build() looks like this:

class FavoritesBlock extends BlockBase
{

  /**
   * {@inheritdoc}
   */
  public function getCacheMaxAge()
  {
    return 0;
  }

  public function build()
  {

    $uid = \Drupal::currentUser();
    if ($uid->isAuthenticated()) {
      $flag = \Drupal::service('flag')->getFlagById('favorites');
      $count = \Drupal::service('flag.count')->getUserFlagFlaggingCount($flag, $uid);

      $test = "Events added to favorites: " . $count;
      $link_url = Url::fromUri("https://my-site/my-favorite-events");

    $text = Link::fromTextAndUrl(t($test), $link_url)->toString();



    }
    return [
      '#markup' => $text,
      '#cache' => array(

        'tags' => ['config:node_type_list'],
        'max-age' => 0,
      ),
    ];
  }
}

I also came upon the EventSubscriber which seems to be triggered exactly when I want, meaning on flagging/unflagging but I don't know if it is useful in this situation, I haven't much experience.

How could automatic block reload be achieved?