rodent
Welcome to the forum!
I agree about the throat erosion. It's called overbore. The 22-250 and the swift both suffer from it and they hold less powder than the '06 case.
The smallest I have seen off the '06 case is 6mm and that is pushing it.
I think wildcat rounds are a cool idea, but the 22-250 will do anything you need in the 22 cal. As far as the 204 goes, it is a good idea but after 400yds it will not compare to a 55 or 60gr (or heavier) 224 pill.
I'd say if you want real long range get a 243 and use 55-60gr bullets pushed to 4000.