You need to let them run longer. You could have had a MUCH more developed trunk right now if you had pruned less often. Acer responds very reliably to a chop, so it's OK if you let them just thicken up first. In fact, when you prune too soon, it can be difficult to ever get the trunk thickness you'd like (the leader has to catch up again before it starts to impart thickness).
Here are a few alternate paths you could have taken:
- From the beginning, if you had planted it in the ground, you'd have most likely had a thicker trunk than you do now in 2015.
- If in that 2015 photo, you had just let the branches run at least another season or two, that would have made a big difference too.
- An alternative approach from that October 2015 photo would have been to shorten the dominant branches by about 30-50%. Just cut them back to another viable branch, and let those branches run another season. That allows the lower stuff to catch up a bit, while still letting the dominant branches do their thing and thicken the overall base. This tends to be my preferred method for early-stage stuff. Let it run, shorten a bit, let it run again, shorten it a bit. Keep everything in balance. By leaving a lot more behind, you leave more available material that can make an impact on the trunk. Only when a branch has had the maximum impact on the trunk do you remove it. Keeping the growth in balance allows more small branches to thicken up, which tends to make for a much more interesting trunk. This method allows you to make reasonably fast progress even in a pot. Treat the entire thing as a working system and you can practically sculpt these trees into whatever form you like.
Not crying over spilt milk here, just highlighting some alternate paths for the next time you get to those stages.
The nice thing about what you did do, is that developing in small pots like this tends to yield nice nebari. Mission accomplished there. But you gotta let this thing grow now. Let it scale up for the next few years. You can balance prune, but stop hard pruning it for a while. And I probably wouldn't even consider touching it with shears until this time next year unless something is going to cause a problem. But even then, you can very precisely regulate the effect a growing branch is having on the trunk by just pruning the growing tips if it's starting to dominate things. People tend to prune way too much, way too early, and it's just not necessary.
Grow this into a bush, and you'll be left with something more interesting to pare back to the next time you do hard prune. Here's a challenge. For the next few seasons, work whatever growth you get into a canopy as if that were going to be your tree. Wire the branches just in case you end up using any of them later. But mostly, just build a canopy and let it fill in and gradually expand. You'll have a lot more options to work with, and you'll be able to spot a trunk line and let that run, which will give you a nice trunk very quickly. Within about 3-5 years, you'll have something very nice to build off of.
Here's one of mine where I did a chop and re-grow that will maybe give a clearer picture for what I mean. If you follow the links in the thread, it goes all the way back to 2010 when I first chopped it. I can tell you now, my one regret is that I didn't let it grow for another season or two before the chop because now it's a much slower process to thicken the trunk. It's actually due for a complete styling in the next few weeks, but it's definitely evolving towards something interesting. The point being, in 2010 I chopped it back to almost nothing, and in 2017 I have a pretty substantial tree. It grew in really well last year, so I have a ton to work with. Within the next few years I'll have all of the major branches in place, and it's just refinement from there.
For yours, I'd focus on healing the cuts and developing a solid base before worrying about anything else. Just let it run until you've healed over the cuts and gotten the thickness you want. Then, one good chop and re-grow from there. I could see
maybe letting the tiny shoots take over this year, and then possibly removing that thick branch and letting that one heal over too, but it depends on where you see the trunk going. I could just as easily imagine ignoring this for a few seasons, then chopping
below that big branch, and building on that as your foundation. But I think if you at some point do remove the big branch and let it heal, then chop to a point just
above where it is now, you might end up with a more interesting base. Lots of possibilities ...
If you haven't read it yet, pick up a copy of Peter Adams' Bonsai with Japanese Maples. That's been my go-to resource for working on these.
I have a number of different types of JM, and while some of the cultivars have some really interesting qualities, none that I have found are quite the living Play Doh that standard acer palmatum is. But you need to let them run to experience that.
Hope this helps. Let me know if any of this doesn't make sense.
Cheers
EDIT: Also, fwiw, I have found that using the clay-type cut paste on large wounds yields much more reliable results than not using it.
I documented some results here. This example uses bloodgood maple, but I've observed very similar things with standard acer. When you have a more established tree, and you're doing refinement pruning, cut paste becomes a lot more optional in my opinion. I like the type of healing that the clay paste encourages for large cuts. I know you can get similar results without it, but the risk of die back is much higher.