BunzI,
After the leaves harden off you can do some moderate pruning to direct the tree's energy away from regions that you don't want thickened. Most, if not all, branches won't be used in the final design as they will be too thick and straight by the time the apex of the trunk is complete. You'll probably want the thicker branches lower to the base and as you move up the tree, branching is thinner so that branching and trunk taper up and out. The ramification and taper of the branches is achieved by clipping back selectively and then wiring (new shoots) in the direction you want it to grow. These tridents keep wanting to grow upward so need to be pruned back hard on top. I realize now you want a straight, more formal trunk but I don't think you want a telephone pole with shoots coming out. I think if you keep things under control this season, you can do more major work as spring rolls around next year. Determine your final height and look and go from there. I don't know if I would want to go any taller than shown here without a bigger base on the tree. With that said, I think one can say that you would need to do some heavy pruning next spring to get some pleasing taper into the apex of the trunk so that it isn't such a sudden jump from thick to thin.