I went into "system information" on the Mac, and under "Hardware" the Zumo is identified under "USB."
So the Zumo is acknowledged by the Mac, but when trying to export from My Route app, the MRA connector still cannot find the Zumo for some reason.
As was said earlier, bearing in mind the phone transfer works, it is probably not worth persevering with this
"Corjan has been busy improving GPX exports alongside the RouteXpert Library lately. Straight lines are no longer shown in the Garmin, 1.1 contains formation and viap points. In addition, the new 1.2 has been introduced that supports track navigation based on viapoints. There will be an extensive blog by Michel later.
In addition to the RouteXpert Library, Corjan has recently been working on improving the GPX export. Straight lines are no longer shown in the Garmin, 1.1 contains formation and waypoints. In addition, the new 1.2 has been introduced which supports track navigation based on waypoints. There will be an extensive blog about this later by Michel."
I can maybe now see an advantage in using MyRoute to create a route on an iPad and then inject it into the BMW Connected app, then into a decent TFT screen.