Hi David, at risk of sounding like a smarty pants you have answered your problem. Feedback controllers and coreless motors are not a happy marriage. All I have read says do not use that combination. As you know Bantry was 4mm scale and the locos came from very varied sources, Hornby, Bachmann, 00 Works & Kitbuilt. All DCC fitted, some sound some not. Never had any problems fitting decoders in the body's just insulate them well. As an aside, i was in our local DCC specialist today to price up having Dcc and sound fitted fitted in a very small tank engine, it was deffo a HOW MUCH!! moment.Price had near doubled since I last had one done. So it will be diy and silent running in future! One more thing to consider is being able to switch point frog polarity and if you need to modify the points.
If I sound like an expert I'm not, just info from experience.
Regards Mick