I got the new board installed and checked, it did not work. I double checked that the solder is making connection to the board from the cart connector. (vias will be covered in final version)
When I used BRAM tool, it reported no BRAM. When I used TurboGrafx-16 CD System 2.0 card to access memory backup, I got corrupted display instead. When I used PC Engine CD 2.0 card for the same thing, I was able to enter BRAM area to delete and format but the text is corrupted.
I need to find my microscope to verify all the pins on HuC6201 is soldered and not accidentally floating. The game I selected at random played fine with the board installed so there's no interference. If those microscope check didn't find any error, I'd need to try a second HuC6201 chip I have to rule out defective chip.
I really wish there were more documentation on HuC6201 beside the pinout, it'd be so much easier to troubleshoot this or clone the chip with FPGA or something similar.