GP2040-CE VS firmware-pi-rp2040

If you're making a home made arcade stick, check out the GP2040-CE project - https://github.com/OpenStickCommunity/GP2040-CE

It's got low latency and good compatibility for a bunch of platforms. Even some compatibility for PS4.

My advice is to rework your mixbox, as it is pretty much the only long-term solution. Personally, I would swap out the keyboard microcontroller and instead wire your keyswitches to a PiPico running the GP2040-CE firmware. That way you can easily remap the keys, as well as select between your controller acting one of these:

Talking about RP2040 powered controller firmware. GP2040 is an amazing project, which is used a lot in the Fighting Game Community where custom controllers are common. https://github.com/OpenStickCommunity/GP2040-CE

you can connect to it the same way as a brook pcb (without soldering), and its firmware is far more customizable with great input latency test results. you can read about the firmware here: https://github.com/OpenStickCommunity/GP2040-CE

congrats on the build & thank you for sharing your experience! if I may suggest you to take a look at the open source project using the Raspberry Pi Pico and other boards based on the RP2040 microcontrollers. https://github.com/OpenStickCommunity/GP2040-CE

In theory, one would only need to use the Pi Pico (Using the CP2040-CE Firmware), since many play fighting games on PC; At least, that I am aware of. This, along with a few other things, would make it an "okay" controller at best, IMO.

I suspect GP2040-CE will pretty soon have some capability to switch between profiles on the fly, though of course you can never predict these things with volunteer open source development. In any case, I'd recommend GP2040-CE as an option over the Brook. You can buy a Pico breakout such as this one for cheaper than the Brook & have access to all sorts of cool features now and in the future.

What upgrades would you recommend as an improvement on this? - Use keyboard layout editor to design a backplate and pay a local lasercutter to cut a metal sheet for you. I'd recommend buying a pre-made bamboo storage box for a few £s and use that as your box. If I didn't already have the Arduino, I'd get a Raspberry Pi pico clone and use better premade firmware.