@subs Thanks for sharing this information. I read that page about keyboard mapping and reconfigured as such via Pimoroni's 2.x update. I even later tried a bit of gamepad mapping for joystick and action buttons, with keyboard for the menu functions. Despite the visual confirmation of their mappings in Recalbox, the buttons (especially action 1-6) didn't respond in-game with all emulators.
The fact that the linked custom firmware exists (and works well), gives credence to the fact that this is more of an incompatibility issue with the Picade board's configuration flexibility, than it shows any issue with Recalbox.
For my purposes, everything works perfectly.