windblownhermit

All the following may just be as a beginner's prejudice from only looking at "classic" bonsai example, so take it with salt.

I feel that you have a nice thickness of trunk but too much of it vertically. Hence, though you do say it is a younger tree, it does not seem to present the illusion of age and mass; not a compelling presence I guess you could say. In that it represents a sapling, which it basically is!

I think, as a beginner who has never done this personally but knows that there are lots of people out there to jump in with more technical advice, that if you agree that the trunk could be shorter then maybe you should height(air) layer it. Of course, I don't know how this species of tree takes to layering, but you would solve two questions with one procedure: a sapling-looking trunk would magically begin to appear older and more massive, and a slightly quirky root pattern will be left in the original tree while a handsomely radial pattern will have grown on the new one. Not only that, but you might get two nice trees out of one.
I guess the drawback would be that you end up with a smaller bonsai height-wise than you may have been wanting.