I don't think previews will affect the crowds that much because it's only Platinum AP. Most locals are Gold AP. Schools are back in session or very close, so no one is really vacationing. Galaxy's Edge still won't be open for a week.
Touring Plans crowd calendar puts 8/20 and 8/21 at literally the lowest possible overall crowds of the year: a score of 1 out of 10. Though, they do list HS as a 4/10 - which is still ridiculously low.
I think you're going at the perfect time.