It's been a little while. Master's degrees are hard work - who'd have thought!

Anyway...in anticipation of tidying everything up a bit, I designed a PCB. I've never done this before, so the learning curve was steep. An Arduino Pro Mini does the brains, and pads for directly soldering on a stepper, or for a connector for a remote mounted stepper. An identical connector takes the cables to the PI. I also added some diodes for flyback protection. This is what it looks like:

So off to my local makerspace (somakeit) to have a crack at making it.