The Arduino Pro Micro is cheap (~3$) and perfect for small boards (less than 81 keys). With a little hack though, it can drive a keyboard with up to 100 keys.
Quickpin
RP2040 Pro Micro 16MB/128Mbit USB-C Controller – Keebio
Pro Micro ATmega32U4 5V 16MHz Micro-USB Development Module Board with 2 Row pin Header Compatible ard-uino Leonardo Replace ATmega328 Pro Mini : : Computers & Accessories
AITRIP 2PCS Pro Micro ATmega32U4 5V 16MHz Type-C Development Module Board with 2 Row pin Header Compatible arduino Leonardo Replace ATmega328 Pro Mini : Industrial & Scientific
40% Keyboards: Pro Micros
Arduino Pro Micro 32U4 - A2D Electronics
USB-C Pro Micro issues
Arduino Pro Micro 16MHz/5 V – OpenPipes
Arduino Leonardo Pro Micro Atmega32u4 5V 16MHz – Maker Zone
Using 24 inputs of an Arduino Micro? - Microcontrollers - Arduino Forum
The micro usb of my arduino pro micro rip off, so I fixed it. What do you think? : r/arduino
