How on node forms do I show weight of all menu items?
Doing a hook_form_alter on the node edit form should do the trick:

  use Drupal\Core\Form\FormStateInterface;

  function MYMODULE_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  /** to appear on the node/add/page form, change this or add more for other content types **/
  if ($form_id == 'node_page_form') {
    $menuItems = $form['menu']['link']['menu_parent']['#options'];
    $menuItemsWithWeight = array();
    foreach ($menuItems as $menuKey =&gt; $menuValue) {
      $storage =  Drupal::entityManager()-&gt;getStorage('menu_link_content');
      $uuid = ltrim(strstr($menuKey, ':'), ':');
      $uuid = ltrim(strstr($uuid, ':'), ':');
      $link = $storage-&gt;loadByProperties(['uuid' =&gt; $uuid]);
      if(count($link) &gt; 0) {
        $link = reset($link);
        $weight = $link-&gt;getWeight();
        $menuValue = $menuValue . ' ('.$weight.')';
      }
      $menuItemsWithWeight[$menuKey] = $menuValue;
    }
    $form['menu']['link']['menu_parent']['#options'] = $menuItemsWithWeight;
   }
  }