The neck size should be set by the expander ball as the case comes out of the sizing die. That assumes the sizing squeezed the neck enough that it has to be expanded slightly. It sounds like the thinner necks don’t get squeezed enough (which you already figured out).
Adjusting your sizing die for the different types of brass would be a big headache. You might want to find a sizing die that uses bushings that can be swapped out easily or buy a second sizing die ($
$).
Let us know how you work this out.