Currently, the only way to manage what you want is to remove the
Auth::routes();
From the routes/web.php and add each routes one by one.
You have all the routes Auth::routes() adds here :
vendor/laravel/framework/src/Illuminate/Routing/Router.php
in the auth method.
Just copy paste them all in your routes/web.php and remove the one you don't want (the GET and POST request on /register).
As for registering the users in an admin panel, there are several ways you could do it. Here is one :
You have a bool on your user model which I would call is_admin.
You create a middleware isAdminMiddleware where you check if the user has the is_admin property. If he has it, you can go further, otherwise just go back to the previous page or return a 403.
In a UsersController for example, create a GET method, which returns the view containing the form for creating a new user.
In the same controller, create a POST method, which adds the new user to the database and makes proper validation.
Don't forget to protect both routes with isAdminMiddleware so that users can't go to that page.