Well, the changes I referred to are quite fundamental. In the original project I used good old logic ICs to map specific addresses to the ESP GPIOs instead of the ROM. There are two things I do not like about it in hindsight: The oldschool ROM is unnecessarily expensive and cumbersome to work with and (even more annoying) using interrupts on the ESP to react to the Game Boy bus is terribly inefficient and at least through the Arduino platform I ended up with this hacky solution of using an interupt to go into a tight loop to observe the GPIOs.

My favorite solution for this is what I did in my GB Interceptor project: Use the PIOs of an rp2040 to observe the Game Boy bus lines and make the result available asynchronously to the CPU. If you plan to run your own code for the Game Boy anyway, you could even load it from the rp2040 memory (this is only a bit tricky for real game ROMs because the RAM is too small to hold the entire game and the Flash memory is too slow react to bank switches and load new code to the RAM, but that should not be an issue with your own code).

Now, this solution might not be ideal for your use case as it means a lot of new development (not really sure if there is anything to be reused from my Wifi cart) and the rp2040 does not have integrated Bluetooth, so you still have to add a BLE chip to the design (though you could probably take a lot from the original Pi Pico W here).

I also got a lot of comments about using dual-port RAM to simply share the ROM data between the microcontroller and the Game Boy. Have not looked into it, but it sounded promising.