Original post has been updated with a pretty solid build guide by @dubesinhower , this is a solid resource for anybody looking for an in-depth build guide for the Ashida.
This is not due to the lack of an MX as those games work without issues on my personal units. This seems more USB related, can you provide pictures of your wiring?
[EDIT]
Black screens can also be caused by corrupt virtual memory cards created for each game in Nintendont, delete them and test...
The pads are only partly damaged so it will be fine. You could always solder to the traces or other pads. The cartridge connector looks a little beat up though and probably best to replace it.
All you have to do is flash it to a Pi Pico (literally connect Pi to PC via USB, drag uf2 file onto the Pi and that's it) then wire buttons and joysticks to the Pi Pico's pads just like what you're doing with your DC controller PCB. It has 8 VMU's built in, rumble and other features. Very simple.
Have you taken a look at using Maplepad? (developed by @YveltalGriffin) it's a great DC controller emulator that only requires a Pi Pico and will even allow you to use an OLED display for the VMU.
Here is the first dirty release of the Wii undervolting "guide".
This isn't an official release yet because:
I'm not really sure how to structure this information in a forum post yet.
I need a larger sample size of systems and tests to really dial everything in.
Brief synopsis/overview...