I can sort of make a matrix of problems, advantages, limits with the Bafang MD's, the DD Goldens, and the geared Macs. The mid-drives have wear, chainline, and shifting issues. The Macs are fat, but light, so disk clearance problems are common. The Goldens are thin and heavy, are shipped with rim and disk capability. But 20 pounds is a lot weight.
I think customer service in DIY is getting a lot better. With my Mac there was rubbing. I figured I needed a disk spacer and just couldn't find one.
@Court J, with his machine shop, sent me a couple. I tried the thin caliper and it's pretty basic. Paul at EM3ev didn't quite lay out the issues, when I bought the motor. Paul had spacers, but slow shipping from Hong Kong. I think one guy wanted $15 for 1 mm or .5 mm.
Once you get it done, it's done, but issues like this tend to scare people off.
I wonder if this guy is in the UK and could just go to the shop? I bought some tires from Chain Reaction, Marathons, and the price was great, fast shipping. Glad he could find the part.