(grin) The usual advice is to put it into the ground or a growing box or raised bed, and grow it out. It'll take a while otherwise. There are several schools of thought on when to prune, etc. if you do this, so you need to do some research if this is the road you want to take.
The very basic way of thinking about this is that the trunk enlarges based on how much foliage there is, and between foliage and root growth you will get a larger trunk. Unrestricted root growth (in a big wooden box with good drainage, for instance, or in a good ground location) on top of a tile so that the roots spread nicely outward, will give you a better chance of getting a nice thick trunk. The foliage could be allowed to grow out, or it could be pruned to develop more, tighter branching.... but the very basic idea is to give the tree room and incentive to grow. Little bonsai pots don't give the tree the kind of growth you need. They restrict root growth, their dampness and temperature vary widely because of a smaller soil volume, etc. (I've even begun to plant several small trees into one big grow box, finding that the more stable moisture and temperatures and soil area benefit them in that way. Sort of in between a raised bed and a growing box for an individual tree. Maybe they like the company! They have to be compatible, though, or the same species)
Think of the bonsai pot as the frame around a finished picture. You only go to the pot when your tree is nearing what you want from it. Just as you wouldn't choose a frame before you paint the picture on the canvas, so you don't pot the tree into a bonsai pot until it is nearly complete, and the final stages are at hand.
There will, however, be more ways to think about this tree and more advice that people give you. Choose what you think is best and learn from it.
Joanie