Hi Mike
.. looks like a regular benjamina to me. You've got a good vigorous start on a banyan (tropical broom..) style.
For me the trunk size dictates the height of the tree, the line through the canopy in the pic below is about where I would have the finished height, shorter is better. I would train the lowest branches down somewhat to help spread the foilage outward and prune the top back hard (do this in the spring/summer .. you can pune it back a few inches now but do the major work in its vigorous growing cycle).. leave some leaves on the branches which remain. The benjamina needs tropical environment to total defoliate so it is not a good ting to do if ya live in Oregon...
This will get the canopy beginning and to fatten the trunk and to eventually encourage aerial roots, Mittar's suggestion is right on .. a grow box (shallow, not more than 2" and larger than the desired/finished canopy size) will help. The ficus like to extend their roots shallow instead of deep.
Your tree looks very healthy .. Do you have a indoor grow area to move it to in the winter? What is the top dressing you are using?
Or maybe that's your soil mix??
Loads of info here, and the internet on tropical bonsai..
Jerry Meislik grows tropicals, also in a cold climate ..
'ere's the link
http://www.bonsaihunk.8m.com/