The Lavender is beautiful but I worry about bees. I was about to plant sage near my walkway....bought about 20 plants, but got stung today and am rethinking this.
I second the walkers Low catamount, one of my favourite perennials for long bloom time and also tough as nails...there is also nepeta Blue Cloud for smaller flowers and its close relation nepetilla. I love that you have created gardens right up to the sidewalk, our cities really benefit from the beauty and ecological impacts!
Have you considered Rozanne geranium or Walkers Low catmint?
How fun to have a poll. I picked the lavender. Providing it would grow there, I think it would give you long lasting blooms, a bit of a free form look, but without being untidy. In my hands, Scabiosa and Johnson's Blue, can sometimes get leggy and unsightly as they wane. Look forward to seeing the results.
I have the same problem with Salvia. I like it, plant it, and them I'm unhappy with it. I really need to stop buying it and get something else. Lurie Gardens seems to have the most perfect "river" as you call it. I think there it's all about the space and sheer number of plants they have.