We went last year and the only park that was open until 9pm was Epcot, and that was for the World Showcase. The other parks closed around 7pm (except for AK which closed around 5pm). It wasn't as bad as I had thought, though, as the crowds were very light and we were able to see and do everything we wanted several times during the week we were there.
The only thing I'd be careful with are the Magic Kingdom hours. I don't know if you'll be able to follow those carefully with Wishes being a semi-regular show.
Although, if you're going in September, you needn't worry about making Priority Seatings right on the 90 day mark. (Except for Cinderella's Royal Table - obviously! ) The crowds are so light that you shouldn't have any problems making arrangements once the park hours are released.