Q.white flowers for hill
I’m planting white creeping phlox and would like to have something white to bloom after the phlox stops blooming. I have a large hill and am concerned about soil erosion all the while keeping the white flowers.

Is this a sunny or shade location? Are you only looking for one other planting? Are you planting on a hill, or is the hillside something that would drain into the garden area?
For a shady area, ground cover I might recommend Sweet Woodruff.
White Lobelia would be nice in a sunny location.
Here are are few articles for you.
https://www.gardeningknowhow.com/edible/herbs/woodruff/growing-sweet-woodruff.htm
https://www.gardeningknowhow.com/special/spaces/creating-all-white-gardens.htm
https://www.gardeningknowhow.com/ornamental/flowers/lobelia/lobelia.htm