How can I have an admin only non-dashboard page in my WordPress plugin

You’ll need something like this:

function your_callback() {

 if ( ! current_user_can( 'manage_options' ) ) {
    return;
 }

 // add your code here to display the menu for admins only

}

Basically you need to check what capabilities the current user has and then select one which only admins have

List of Roles & Capabilities
https://wordpress.org/documentation/article/roles-and-capabilities/