If it were mine, I would get into the root mass, remove any downward-pointing roots under the trunk, comb and otherwise ensure that the surface roots all came out radially and didn't cross each other or dive back toward the base. I would repot it in a container that was shallower than the present one but with at least the same volume of soil. I don't think it's time to constrain growth by containment just yet. You want it growing vigorously so that the big cuts heal rapidly and you get plenty of budding. To restrain the size, I would progressively cut back the existing large branches -- basically aiming to have a whole new crown of branches that are smaller in proportion to the trunk than what are there now, and mostly more horizontal. If it's growing vigorously, you should have plenty of buds and new shoots to choose from. More than you need, in fact.
This is pretty much my program for my similar trees, and based on experience with earlier ones. I'm in coastal California where we have a big long growing season, but I think the principles should hold where you are as well.
And I'll be happy to hear anybody's critique of what I've said, because I could be off-base. (It's happened before!)