how would i event a custom menu
The short answer is: You won't.
Please don't torture yourself.
But you may ask why: Creating tech with events will produce a workflow for you aswell as a result for your players that is worse in every point than writing code. Not just a bit, it's an entire different level.
Maps and events are a content-managing environment inside the game's map-gameplay-logic, that means they're designed to help you "easily" bring your ideas of what to put into the game's playtime content-wise. All the menus aswell as the battle-screen's functionality are programmed outside of this and got open-for-edit since RPGXP for a reason. The attempt of recreating this inside map-logic is a thankless task that is never worth.
Everyone. Please. Don't let yourself get scared from the fact that programming by writing code is the better and (I know some can't believe this) way less stressful option to follow when you're about to create systems, even if you don't have any experience with coding yet.
Actually, experience with events may have already teached you a lot about proper use of math/logic in procedures. It's a good opportunity to start.
Ruby is a scripting-language that allows you an incredible ton of things and freedoms of choice for multiple ways to achieve something. It is a perfect match for (but not only) beginners. °▽°
~炬燵あ