Cornish Cabbage Plants

I don't think either sand or rock dust will have much effect on the pH of soil. I've used rock dust in the past but don't see the point in it anymore, it can take decades for the minerals to be made plant available. If you haven't done a soil test you've no idea what your soil could be deficient in. Even if there is a deficiency there are much better ways of correcting it than rock dust.

You can definitely move brassicas but I think the stress of disturbing their root system like that would make them more likely to go to seed. When I've done it I try to keep as much of the root system intact as possible.