I couldn't find my breadboard, but managed to dig out an old Arduino Mega with a prototyping shield. That should do the trick ha ha!
The Mega can act as the slave for now. On Monday I'm going to solder some headers onto the protoshield for the Wemos D1 mini and also the Pro Micro, and this shield can serve as the test unit for this project and many more in the future.
I've been having a bit more of a look at the case and where to fit my components. For the cartridge slot I will fit the Nano and its breakout; it's usb port exposed for serial use and programming. I will need to remove the 2x3 pins on top of the Nano and potentially shorten the pins on the bottom to get it to fit nicely. For the breakout pins I have some right-angled headers from my Pi Zero kit to use. If I can get the nano and header soldered nicely onto a bit of perfboard and make it fit it could even be modular.
I have a plan to fit the Pro Micro above this board and inline with the Link cable socket to keep the case looking clean on the outside. There will be a bit of plastic to be removed to allow the board to fit in flush. I will need tomake sure the screen doesnt get in the way when it arrives before deciding on that.
I have also decided to drop the start/select buttons and swap them for status/mode LEDs.