Beautiful job! That screen is so bright and sharp. You just put a tease for all that do not like the GBA SP due to the closeness of the buttons. The regular GBA buttons with wider placement looks minimal compared to the Big Boy Advance!

I play a SupaBoy (portable SNES) and one of the problems is having to hold the screen straight to keep a direct bright view. This is due to the screen having a minimal view angle. Your Big Boy looks to have a great view angle range evidenced when you tilted it.
Another tease for you. In addition to adding video output for the big screen, maybe throw in a NES controller port? This will give the option of setting the unit down and just having to hold the gamepad, useful especially if you add that video output.
My Supaboy has video output, but the SNES controller ports only go "hot" if the video cable is plugged in. The video connection port has a bypass switch inside detecting when the video cable is plugged in. The Supaboy PCB can only work one set of controllers at a time due to maintaining the original port connection with the external gamepad chip. It would be nice if the gamepad ports are always on, would make the Supaboy easier to play at home even when not hooked to the big screen.
If you did add NES gamepad mod, I imagine you could only enable one set of buttons at a time on the Big Boy. To keep both hot, maybe go the
direct wire method to the gamepad buttons, bypassing the controller chip inside the NES gamepad? A nine pin connector (eight buttons, one extra for other side of button contacts) could make this work to allow easy unplugging the modded NES gamepad.
Wired with standard 2600 controller port layout this will also allow all 1st and 3rd party 2600 joysticks (no internal chips have direct wire) to work with single (A) button GBA games!

