How to utilize custom form modes to edit a user entity?
You have to add the form mode in the handlers section of the user entity:

/**
 * Implements hook_entity_type_build().
 */
function mymodule_entity_type_build(array &amp;$entity_types) {
  $entity_types['user']-&gt;setFormClass('my_profile', 'Drupal\user\ProfileForm');
}


Use the standard class ProfileForm as you only want a set new form mode, not a new form class.

Then you can use this new handler in place of user.default in the routing file:

'_entity_form' =&gt; 'user.my_profile',