Menu system without libraries
Interactive menus are certainly not a "simple" task. That is why people use libraries.

Whatever you try you will end up with complex code - either long winded with lots of duplication, or somewhat cryptic with lots of pointers, arrays, and structures. Not an easy task for a beginner in C++.

How I would set about it would involve the following things:


A structure that defines a menu entry, including pointers to one of: sub-menu array, numeric variable (including max/min limits), option array, function to execute.
Arrays of the above structure to define each menu
Array or linked list of "breadcrumbs" through the menu to track where you are and go back up properly
Functions to display and navigate the menu arrays on an LCD.


All of that uses fairly advanced C coding techniques, especially when it comes to managing all the pointers, etc.

The "simpler", though more long winded way, would be to write individual functions for each menu and sub-menu, but that would make your program incredibly long and prone to have errors when duplicating (copy/paste) code.