The Syzigium trees would grow plenty tall if not trimmed, so it was the shape or form you didn't like?

I may be all wrong about this, but with the tall trees beyond, and the many small trees in the foreground -- that appears to be a lot of roots sucking up the moisture. Anything you plant could suffer from root competition from those established plants. My guess is the last thing you need are more trees. But -- it's difficult to really grasp the complete situation from this one photo.

My rambling thoughts: you have a very attractive stone wall, and you have the impressive towering trees behind. What about some sort of free-form shrubs with interesting foliage to go behind your developing topiaries? The contrast of red or maybe purple foliage could help the foreground greens be more defined. And if the colorful shrubs are not so tall, you'll still be able to appreciate the strong vertical lines of the tree trunks. Personally, I'd take the xanadu over nandina along the top of the wall any day. Nandina is so stiff and upright for that location; I like a trailing plant for along the top of a wall.

You are in tropical Queensland? With a wet and a dry season? I'm not so well versed in plants for that part of the world.

Your slope looks fine to me. What do you not like?