Take my advice with a grain of salt as I am still pretty new, but the first things that come to mind for me are:
For trunk thickness and taper I would plant it in the ground of a much larger oversized pot and allow to grow freely for a few years while keeping up a good fertilization schedule.
Surface roots can be improved by wrapping wire around the base of the trunk below where you want your roots to be improved. Plant deeply and allow if to grow in the ground or a larger pot for a few years to develop a new root system. Also, you could plant them over a piece of tile to improve the radial structure.
Leatherback linked an article he/she (sorry I am new and don't know so no disrespect intended) wrote here:
www.growingbonsai.net/nebari-creation-with-young-plants/
I hope some of that helps you out in some way shape or form. By the way, they both look healthy so you are definitely doing some things right. How long have you had them? Good luck!