Nice stump. Definitely give it a couple years to recover, you want it growing vigorously before your next hard chopping back.
Photos can be deceiving, but your grow box looks deep, maybe 6 or 8 inches.
It is potted up, don't do anything more with it, but should you dig another stup like that, saw off the bottom flat, even if you have to sacrifice a few more roots, it is better to use a wider, flatter grow box. Aim for less than 5 inches deep. Reason is that it will be difficult to fit this into a bonsai pot. You are ''locked in'' to growing for a couple years and then doing the flattening out of the root system, let it recover 2 more years, then put in bonsai pot. If you had gone with a shallower grow box, you might be able to make the move to a bonsai pot in one step rather than 2. But it is too late with this one, so keep it in mind for the next one. No matter what you have 4 or more years of work ahead of you on branch structure before you will be ready for a bonsai pot, so you are not really loosing time with the root work.