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.
SuperMini nRF52840 Pro Micro Bluetooth LE (BLE) Controller – Keebio
Buy Pro Micro - 5V/16MHz - Atmega32u4 module - Botland - Robotic Shop
OSOYOO Pro Micro Board ATmega32U4 Leonardo 5V/16MHz Module Board Type –
SuperMini nRF52840 Pro Micro Bluetooth LE (BLE) Controller – Keebio
ATmega32u4 Pinout, Specification & Datasheet
ATmega32U4 Pro Micro Controller Board for Arduino Pro Micro Type-C USB 5V US
Maple Elite-Pi Download
Arduino Pro Micro – Voltaat
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
Pro Micro Atmega32u4 5v16mhz Replace Atmega328 For Arduino Pro Mini With 2 Row Pin Header For Leonardo Mini/type-c Usb Interface - Integrated Circuits - AliExpress
Adafruit KB2040 controller: Pro Micro size, awesome RP2040 power #Keyboards #RP2040 @KbdNews « Adafruit Industries – Makers, hackers, artists, designers and engineers!
Arduino pro micro pinout instalación - Pasión electrónica
Arduino Leonardo Pro Micro Atmega32u4 5V 16MHz – Maker Zone
