Replaced the cable and the computer still isn't seeing the SSD.
I think I'm going to bring it to L2 for a diagnosis. No idea how else to proceed, since checking the integrity of the cable and SSD are beyond my little shop's limited capabilities.
In any case this has been a fun learning exercise. I also have a couple of Xboxes that suffer from red circle of death syndrome. I was reading up on it a couple years ago and it turns out that there are some weak heat-sink solder joints. So I might tackle those in the future.
But for now, I think I'm going to have to take this MacBook to a professional. I did my best, and it wasn't good enough