so, a few things I'll note, since you asked
I have removed much of the ramification and secondary branching in favor of taper and structure. I'm often bothered by trees with branches that don't taper and that is my taste, for better or worse. I never liked the surface roots on this tree so years ago I started a more radial root spread below the surface roots that you may have seen in earlier images of this elm. I cut off the large surface roots, hence the reverse taper. in time those roots will thicker and the reverse taper will diminish, plus the dead areas will become hollows. its pointless for me to try to preserve the dead wood, in my opinion. also the root mass is paper thin now.
here are some images from the same angles as the winter ones. it doesn't help that the leaves are too large because you really don't notice the taper here. winter images will show that more. you can however see that the tree is shorter and totally healthy despite being aggressively cut back repeatedly.
just to be frank, I'm not posting for 10 years on this thread to be praised. if someone doesn't like it, of course that's fine. personally I think it's on a good path. people get absolutely pissed off that I put developing plants into bonsai pots... I'm sorry to everyone I've offended by doing this.
Well, you can pot trees in whatever you want! However, by putting it into a small pot you are slowing down the growth...a lot. So if you want to grow out a reverse taper or add thickness to the main trunk, that would happen much faster in a larger container, whether it be a wooden box or anderson flat or whatever.
As for your tree, you did write "still not entirely sure how I want it to look in the future". I can see how people would read that and conclude that maybe you're not happy with where it is or where it is going. I see potential for this to be a twin trunk or even triple trunk tree. I've taken your two leafless photos from the spring and worked from those because it's easier to see the structure, though I realize not all of those branches are there now.
The first thing I saw in this photo was that if you were to bring up that left branch a bit, it could possibly work as a second trunk in an upright/naturalistic style. There are 2 problems, that lower branch is right in the front and the main trunk is very thin. I think maybe you could move the front branch over to the right and use it as a third trunk, or maybe shift the viewpoint to the left so it's not right in the front.
But the second photo could be easier to work with as that third branch/trunk is in the back. Here I've moved the right trunk upward to a more acute angle so it looks like a second trunk. Then the hierarchy would be (1) current main trunk is tallest/thickest (you'd need to let some sacrifice branches run wild to thicken that trunk), (2) the second trunk on the right would be a little thinner and lower, and the third or back trunk would be the smallest and lowest, poking out on the lower left. I think the way I've sketched the top is probably a bit taller than it would need to be but just to give a possible idea.
Of course, if you have something completely different in mind, such as a much shorter tree, that's fine. I can only go by what I see in the photos and what I like in trees. But I think something like this could lead to a nice result.
