Using hugel for trees and bushes is always a great idea. In the end it is most beneficial for trees and bushes so it sounds like the tree peonies would really benefit from it. Depending on the orientation too you can get a larger south facing slope that you can plant plants that would benefit the tree peonies while they get established. I don't really know what they need, but I'm sure there are a ton of companion plants that can provide some soil cover, nutrients, and stability.
