For anyone wondering about the end result of all this:
I started by removing some of the material between the inner and outer surface of the pinch point, with no success. Then I machined out the inside of the cassettes with wire brushes and emery cloth, found it much easier to insert the bearings but was still tight at the pinch point. I then split the cassette completely at the pinch point.
I was on track for the next race weekend and the difference was huge. I am now free out of the corners and able to turn much better and accelerate out of the corners again. End result was that bearings unable to twist in their cassettes = bind, at least in a low power class. I’m going to take a bit more material off and polish the inner race of the cassettes up nicely as there’s a bit more I can get out of them.