The actual problem isn't at all making a 4G mini-computer: you can literally buy the necessary parts as modules and wire them together with a a half-way decent microcontroller board and an smallish LED display and then make some code for it in something like Arduino IDE (though I would recommend Platform IO + VS Code instead).
The problem is making it small (especially thin) and capable of running of batteries for days rather than hours.
For example, if you're trying to actually solve the hard part of the problem you would be better of using a micro-controller with an ARM core rather than the ESP32 as those things are designed to use less power. Also you wouldn't be able to use boards as those things usually waste power versus designing your own.
That said, it's a nice hobbyist project.