How to take advantage of the extra pins on a Proton C?

[Surpex]

I'm using QMK to make my layout for a typewriter conversion. The problem is, since I'm building the firmware as if it were for a Pro Micro, and then converting it to a Proton C layout (using CTPC=yes), I'm still limited to the available 18 pins on the Pro Micro and can't use the full 23 available pins of the Proton C. Can someone help me out here? I don't know how to write it natively for the Proton C, so I don't know how to use the extra pins. I've read through the QMK docs and it doesn't seem to answer my question. i'd really appreciate any help.

[Findecanor]

Maybe the extra pins are not supported that way?

I would start with the code in "qmk_firmware/keyboards/proton_c".
In "config.h", define MATRIX_ROWS and MATRIX_COLS to the dimensions of the key matrix.
Define MATRIX_ROW_PINS and MATRIX_COL_PINS as arrays of the pins to use.

[Surpex]

Other than those two things, is it the same process as writing the code for an AVR based firmware? I've heard that they have to be written differently.