Nice project, @Sonicblaston! Thanks for sharing blueprints, purchasing links for buttons and joysticks, and your progress! Looking forward to seeing the final result!
You are probably already aware of this, but instead of going for the kiosk browser mode or trying to get the bootloader to work, on the Raspberry Pis, including your chosen RPi4, you could use McAirpos* to get it to run MCA games natively, also as part of RetroPie, Recalbox and Batocera together with other emulators and games well suited for your arcade cabinet:
(*Given that it still works as it should on the latest releases of these retro gaming systems. I haven’t had time to test and update the repository for a while.)