@gwennelsonuk By the way, Libreboot *is* the upstream, for the pico-serprog version that it uses. More information is available here:
https://libreboot.org/docs/install/spi.html
Riku Viitanen @riku forked Mate Kukri's @mkukri pico-serprog project. By the way both of these people also contribute heavily to Libreboot.
The forked version sets a higher drive level of 12mA by default, also configures LEDs properly that indicate activity while flashing. Every RP2040 device is supported.
We also forked stm32-vserprog.