You don't want to increase the branch thickness. You want thin branches relative to the trunk in order to make the tree look younger. You can, however, improve branch taper by ramifying the branches a bit. Make sure the tree is healthy and vigorous to promote back-budding, and then cut the branches back to interior buds to keep them short and dense.
As for the trunk, you could grow some sacrifice branches to thicken it, but I don't think you need to. I actually like this as a slender tree. Just let the bark continue to develop to give the tree an aged appearance. It would also look good with a thicker trunk, but slender trees are underrated.