Congrats!, I have a tool i created a while back to analyze the signals more easily coming out of the gameboy if you would like one I can send you one? (This is not an advertisment) heres the link to the tool. https://www.tindie.com/products/WeisTekEng/gb-breakcart/ let me know if this can help you out in some way.

cheers.

That's very nice of you! I currently have my wires soldered to a donor board of a Game Boy pocket though, where I've already removed parts like the power switch, cartridge socket and the speaker etc. - so for prototyping I'm all setup! :)

No worries! sorry I was late in offering. Best of luck!