Can I use another entity in the route to provide an entity to an entity form?
You can use a controller with the user as only route parameter. There you can put the code to retrieve the entity that matches the user and build the entity form:

public function myEntityForm(UserInterface $user) {

  // get $my_entity that matches $user

  $form = $this-&gt;entityFormBuilder()-&gt;getForm($my_entity);

  return $form;
}