I'd suggest you post this in the annuals or cut flower forum.

I am in a very different weather zone than you, but my take on it is that snap dragons and zinnia will be mostly over by mid August, unless you start them in the late spring. Lisianthus can take 4-6 months to bloom from seed, so they may not be ready if you havent started them yet. (mine bloomed in September/October the last two years.)

Galardia and sunflowers should be about right for mid Aug. You should look into Dahlias, they are in their prime in mid-late August. There are quite a few different sizes and bloom formations in whites and yellows.