No matter what else you do it will take time. You've helped it along by moving them into bigger pots, that's a good thing. The other is allowing them to go vertical as much as possible; the taller you allow a tree to grow the more it will fatten the trunk and surface roots...but still, over time.

While this is going on you can, of course, pick the front-style what will be the final design and (pretty important) figure out where the apex will be in the future and where you will make the cut to top it when it's ready. Actually, after a few years, you can have it to the point where the final tree has it's branches wired and refined to the point that all you have to do is make the final 'top' cut and finish styling the apex.

By this time you will have raised the tree a few times, continued to expose surface roots and it should be about ready to pot.

Bob