Hiya Kate, Welcome back! We've missed you and glad that you have finally gotten news of move date.
The Knot garden looks like it would be a great project. I've never seen one before so I Googled and looked at some of the images. From the array of images, I noticed some gardens don't necessarily have the "knots". Some designs stay flat. Anyway my thought regarding semps is that there are the mounding types that you could consider that would give more height in their growth than the other semps that gave more of a spreading out pattern.
Sometimes in a project, the most fun is the planning of all the possibilities. Do have loads of fun! and I'd like to see what you come up with.
