I switched to an arduino micro (clone), so to have a micro USB port instead of the mini on the other one. It’s also smaller. I mounted the optoisolator and a resistor (220 ohm) on a piece of stripboard and mounted to underside of the arduino.
Here are some pics:
Next task is to seek power-saving modes for the arduino and find some decent keypress combos to get the GTM into relay mode reliably, and not accidentally take it out of relay mode if it’s already there.