I'd be pretty surprised if the lack of sufficient current is causing your issue, especially on a multi strand harness like that one, so it's pretty slim as a possible problem. As long as all your contacts are clean and not bent it should be fine, and you would have noticed that when you did your continuity checks.
That leaves us with perhaps a component on the PCB? And if that's the case, of course it would be easy to troubleshoot that if there was a way to get them to send some new PCB's and you could just swap them out. Do you think Bafang might co-operate and agree to send a complete set of PCB's rather than having to buy a new motor? That would be a lot easier than trying to go over every trace, and looking up the spec's on every component on the existing PCB and test them, especially if it turned out that some of them had to be isolated to test properly (for example to use the cheap MOSFET tester I bought they have to be desoldered from the board first and I found it to be such a PITA I gave up).
I guess there is always the carpet bomb/lottery approach: buy and replace any suspicious components that are likely to fail in the hope you found the right one, I'll probably do that next time with MOSFET's.