Well, all this depends on what YOU want to do with the tree. The "grow it out" is fine as far as it goes. However, you can make a mame or shohin sized--smaller bonsai classifications) from what you have with a shorter time line.
If you're after a larger trunk, growing it out in a container is doable, but slows and already slow process. Planting it out in the ground for three to five years can halve the timeline for a larger trunk. It will take a decade to get the same increase in size in containers...
Mame and
shohin sized trees use smaller trunks, but they also require more care as everything is small, soil volume (which dries out quickly, less branching to work with, etc.) It's a trade off...
Again, this depends on what YOU want in a "final" (no bonsai is ever really "finished") tree.