Problem is, you have a young leggy tree. It's not what you want in a bonsai which means taking time to rectify. Unfortunately, it's already a bare, boring trunk. I have a few species of Ficus that I am working through this same problem as I only have one of them. Part of the problem, is the tree is not healthy enough (lack of leaves) to give you a suitable enough amount of potential cuttings to start again and it is too thin to air layer.
From my side, I don't see these trees as having potential, so over the past 12 months I have been keeping them well fed, sunned and watered to produce a mass of new growth that I can get viable cuttings from. The tree(s), I will only keep around until I have a couple of cuttings that strike.
The alternative is years (even with a tree of your size) of cutting back and chasing back buds. Ficus have a reputation of back budding well, but it comes with a caveat, you need a healthy growing tree. I've even chased back growth before on a smaller F. natalensis and it's not a one-growing-season job.
To my eye, your tree doesn't look like it's throwing out a lot of new growth. I have 2 rabid F natalensis that are covered in buds and throwing out roots everywhere. If you want anything of a fat trunk in the next minimum 5 years, you want a super happy tree. Also, chasing back growth and getting the tree compact makes this goal harder. If you let the tree get healthy then it is only going to push growth further out. It's just not a good situation.
Personally, if this were my tree and I couldn't get a better F natalensis my approach would be the same -> look after it until it is putting out masses of healthy growth and then take cuttings and start again. I believe you will save yourself 2 or 3 years.
