Pi is inefficient for this task. What you want is a microcontroller, which is fully dedicated and draws less power. Plus there’s more support via QMK, which will do everything you need. Remap keys, change debounce algorithms, add macros, tweak polling rate, cycle RGB effects, you name it.

My advice is to just get an existing PCB and make a custom case. Throw a STM32 in there if you want the best stuff. If you’re dead set on designing everything from the ground up and sticking with a Pi, you’re going to spend months developing the damn thing and it’ll still need an external power source.