Very nicely done! If even some of the parts work eventually, it’s a win.
As for grounding, I rig up the resistor and screwdriver to the grounding wire I have straight to the wall outlet. No chance of frying anything.
One website you probably wouldn't buy from is selling it for $75, even if was available it'll cost you a lot more time and money to reproduce. As the the precious USB cycles thing? Perhaps look out for a junker PCIe G5 with the card - G5s can be found pretty cheaply if picked up locally.
I finally found the problem !
After cleaning the logic board, replacing the capacitors on both boards and two diodes on the analog board (as suggested by @bibilit on another thread), still nothing worked. My voltage was a little bit better, but still not enough.
So I changed the CNY75GB optocoupler, and now everything works perfectly fine !
Thank you all for your help