how to develop hardware like this as a hobby? using a 3d printer?

I have a small hand with hitchhiker thumb, my hand can't stretch, I want to build a narrower keyboard for myself.

One more thing: if you want to do an embedded project, you can use RNBO /for Max/MSP that can export to Raspberry Pi, Web and VST. It has great synth building packages among other things. https://rnbo.cycling74.com/

Max/MSP + Arduino, buttons, a laser cutter and some plywood!