How to define template suggestions for node add and edit form by content type?
I usually use something like this for template form suggestions : (works for all forms, not only entity forms)

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function &lt;HOOK&gt;_theme_suggestions_form_alter(array &amp;$suggestions, array $variables) {
  if (isset($variables['element']['#theme']) &amp;&amp; is_array($variables['element']['#theme'])) {
    foreach ($variables['element']['#theme'] as $theme) {
      if (strpos($theme, '_edit_') !== FALSE) {
        $suggestions[] = 'form__' . str_replace('_edit_', '_', $theme);
      }
      $suggestions[] = 'form__' . $theme;
    }
  }
}