@anneliesgeraets Don’t delete your menus. You’re right, if you delete a menu (or any object, for that matter) it will disappear from all breakpoints. Instead, go to the Layers sidebar, find the menu object, and hide it (choose ‘Don’t Display’ in the context menu). Hiding or showing an object can be done separately for each breakpoint.
FYI: By default, Editor X hides the default hamburger menu on the desktop and displays it on mobile and tablet. It does the reverse for regular menus. You can always override those default choices in the Layers sidebar.