I started thinking about this a couple weeks ago, after being inspired by seeing some the KSP controllers out there.

Currently, I have a list of inputs and outputs for the controller, a bock diagram of the parts and a notional sketch of the panel layout.

I've started doing the schematic capture in KiCAD, and wow am I impressed! So far, KiCAD is at least as good as what I was using back when I was doing board design professionally, with quite good documentation and libraries.  Well done!