Everything I am going to say needs to be done during the growing season. Preferably at the point that the plant is growing well and has produced new leaves and will be producing new root growth/

Without seeing the root base I would first put it into a bigger pot with good bonsai soil. I say this because it has few limbs and is too tall for the size of the trunk.

If the roots looked good and I could find a suitable rock I would try a root over rock style.

After putting it into a larger pot remove all of the branches that are growing from the underside of the main branches. Tip prune all stems to encourage shorter internodes and smaller leaves.

It is hard to give direction when I can not show you exactly what I mean and how to go about it.
Hence this is going to mean a lot of trial and error on your part. Do not worry!! We all kill many trees along the way. It is a learning process and takes time.

I kind of like it the way it is, maybe retain the style it is as a small informal upright and learn some before going on?

In your part of the world you have a limited growing season for tropicals. To keep it healthy and growing inside during the winter you will have to give it an emulation of sunlight for 8 to 12 hours a day and heat/humidity which will be a problem during your winters. You will want to buy a book on growing tropicals indoors and/or search the web for material.

I wouldn't get too excited about arial roots, so far the tree is pretty young and small. They take a good while to produce anyway and they probably wouldn't enhance the look of your tree.

What you do have is a very hardy little Ficus, Retusa is sort of the junk-yard dog of the species. One word of caution: the white sap of this (like all of the Ficus) can cause skin/eye irritation, when pruning the tree be careful not to touch your eyes or the skin of your face.

Bob