Given that you are simply moving a link of an existing route in a drupal menu, you are probably better off using the more simple hook_menu_links_discovered_alter(). A custom link/route is probably a bit of overkill.
But, presuming you do want to go the way you are in your example, you would need to have both a
In the menu links file you might have something like:
title: 'Hello Admin User'
description: 'See a Hello Admin User Message'
and in the routing file you might have:
_title_callback: 'Hello Admin User'
_permission: 'access content'
You've got a bit of a mix going in your example. Note that route names and links names need to be unique across the drupal system. Using your module name as the start is a convention but not required.