Nice work. I did something similar with an American elm I was growing out in the ground that's about 3-4" in caliper. Since it was in the ground, I put a pot around the tree, stuffed the area in the pot with sphagnum moss, and added mulch on top. This year I dug it up and had 360 degrees of radial roots, and dozens of them at that. Makes me want to use elms more given how responsive they are.