I was working this problem concurrently with someone on the #DIY channel of the HOTAS/HOSAS/SIMPIT discord server. He helped me a bit with understanding how the shift register works, and described the different SR pins to me.

I actually figured out the swapped pins this morning while looking at that last pic I posted with both the diagram and picture reference of the board from the AliExpress store for the shift register. The “S/L” pin is their terminology for CS, and CLK is SCK. Except the labels on the picture representation and on the board itself are incorrect, showing those two pins as swapped.

I have only a very basic understanding of what all these pins do, but the fact that I had the same problems with what I thought was the CS (data leg) pin connected, disconnected, or connected to a different pin than specified in mmjoy2, clued me in to the fact that something was off. It was then that I started looking closely at the diagram to verify that things were labeled properly.

Now that it’s working properly, I can quite easily accomplish my cougar throttle switch upgrade project