Good Day! I think a possible solution is to create a separate User Role for the Employee who have Project assigned can be set to this new User Role during Project Assignment. Then add the access to the menu navigation item to only the new User Role. The role should have all access same as Employee user role + access to this menu item and its components.
Hope this helps.
Hi Peter. What you describe sounds more like you want conditional visibility of a page but not based on the role but on the specific user. You can implement such behavior by retraining the access rights to entities of certain type in the security access settings: