My lavender is pretty tall, but I know some of them are shorter. I'm not sure if these are tall enough for you, but they are my favorites.

Purple coneflower, black-eyed susan, and yellow yarrow. The first two are tidier looking and continuous bloom until frost. I am not sure these will be tall enough for you though. The rheubekia will spread and may not give you a tidy line of formality if that is what you are looking for.

Ornamental grasses with billowy plumes would give you height and movement if you don't want blooms.

If you want blooms, Knock Out Roses bloom here in z7 from May until Thanksgiving. They are low maintenance, but would need more water and so that might not be possible out there in your hedge bed. Deer will eat them, however.

Buddleia 'Pink Delight' and 'Royal Red' bloom profusely and have similar growing conditions. I have lavender around a few of my butterfly bushes.