Update for anyone who might come across this later: I managed to get the keyboard and track pad working. From a helpful poster on XDA developer forums (whose original post inspired me to give this all a shot), I learned that I had snipped the red wire in the wrong cable. It was the red wire in the USB-A to Micro-USB cable that needed snipping; I had snipped the one in the USB-C to USB-A instead. The photo was taken pre-correction, so you can see the splice I put into the wrong cable.
Here are links to all the stuff I bought to hook up my Note 10+ to the Atrix lapdock:
There's something wonky happening with the connection between my Note 10+ (running newest OneUI 3.0) and the Atrix lapdock. Sometimes I can plug it right in and start working no problem. Sometimes, the kb/tp aren't recognized and takes some fiddling, whatever combination of which gets thing to work I haven't quite figured out yet.
Any reputable brand does the job. Anker, Aukey, even Amazon's own brand. Look for Benson Leung approval, which Amazon reviews tend to note, but outside of that it's not anything special. You'll find the cables used in the car take extra abuse due to constant bending and heat, so they eventually wear out. I use Android Auto which is way pickier and I found cables fell out of its tolerance range rather quickly, but this one from Amazon has held out the longest so far, ironically far longer than cables with all sorts of woven fabric and fancy designs supposedly meant to improve durability.