This is the fourth time I rebuilt this library from scratch to find the sweet spot between ease of use (beautiful is better than ugly!), testability (simple is better than complex!) and potential for ...

Raspberry Pi Pico's built-in bootloader allows you to write firmware via USB or SWD. However, in some situations, other interfaces may be useful. Picoboot3 now offers the options of UART, I2C and SPI.