Changing the view mode of EntityReferenceFieldItems programmatically
At this point it should be no problem to change the view mode in the render element of the referenced node. The node has yet to be built and will, hopefully, use the modified view mode.

You can try something like this in a paragraph preprocess hook:

function mytheme_preprocess_paragraph(&amp;$variables) {

  if ($variables['paragraph']-&gt;hasField('field_view_mode')) {
    $view_mode = $variables['paragraph']-&gt;field_view_mode-&gt;value;
    if (isset($variables['content']['field_ref_node']['0']['#view_mode'])) {
      $variables['content']['field_ref_node']['0']['#view_mode'] = $view_mode;
      unset($variables['content']['field_ref_node']['0']['#cache']['keys']);
    }
  }

}


Edit:

To prevent that the node gets cached on it's own in the wrong view mode remove the cache key.