First, let me say it looks very healthy - which is not always an easy thing with serissas.
Second, if you are happy with the tree and enjoy it, don't let what I have to say take away from that
I think your main challenge with this tree will be the trunk and the primary branch location. The two main branches are too low and too close together, and you have a long stretch of trunk with nothing in the middle. The tree looks a little bit to me like a topiary - long stretches of bare branch with a "powerpuff" of foilage at the end.
I would start by separating the masses of foilage to add more layers to the tree. Try to break up your apex into several masses of foilage, maybe moving some branches down into the bare space to de-emphasize the bare trunk. Do the same with the two lower branches and aim to creat flat pads of foliage instead of balls. You really really want some new buds to pop on the trunk. If this doesn't happen, you might consider grafting branches on the trunk. I'm not sure how this would work with serissas (I don't have much experience with them).
Here is an example (not a final image but an idea of where you should try to head)
www.bonsainut.com