[SiriusHardware]

I have a Pico too but haven't done much with it as I would rather programme it in 'C'- they haven't got the working environment for that language set up very well yet, although there is a script which sets everything up on the (Linux Computer) Pi if you don't mind using that as your main development machine. I really wish they had not used the 'Raspberry Pi' brand name for the Pico. The Arduino IDE is supposed to be bringing in support for the Pico as well as a few other PCBs which will use the same microcontroller.

My impression (unconfirmed) is that the Pico has quite a lot of onboard RAM and is also very fast so you could do something like the MK2 Arduino version (which currently uses an external SRAM as a serial buffer) but try using the internal RAM on the Pico instead.

If just not having the right optocouplers is an issue for Mark, fear not, I can easily populate the opto positions (if desired) by soldering SMD versions to the through hole pads on the upper side of the PCB - wouldn't add much to the weight and I have more than enough of them lying around at work.

As regards 3.3V vs. 5V drive, the optocoupler LEDs are 'lit' for such a short time during each key press that running them from 5V through 330R doesn't stress them much, so I did not bother to change those values for the Arduino version. The reset opto is lit for longer, but only rarely.