Here's what I would do if I were rebuilding this one:
1. As
@combatwombat mentioned, use chunkier rocks for the spillway. I would also play with perspective, using a slightly wider spill at the bottom, and a narrower one behind it. This gives the illusion of depth.
2. Build back and around the tree, in order to get rid of the volcano effect and add depth. Wrapping it around the tree also gives it a look of having been there before the tree grew. Use larger rocks in front and somewhat smaller ones in back, again to give an illusion of depth.
3. I wouldn't make the whole thing more than half a meter tall; you don't need much height to get good sound and aeration. In fact, a longer, flatter stream/waterfall is better for both, plus it looks way more natural. Between each cascade there should be a small pooling area.
4. Put one rock at the base of the falls, just in front of the waterfall. In Oriental gardening, this is called the "carp rock" and helps to give an impression of depth-of-field, while naturalizing the scene.
5. Plant behind each level, and also behind the last cascade, such that one can't see at a glance where the stream originates. If you have a bog back there, even better!
Lastly, if you put a really large flat stone in front of the pond (as seen from your main viewing area, it will give the illusion that all the rocks are just as large, but just much farther away. You'd be surprised how much bigger your pond and falls look!
Just for the fun of it, here's a visualization of what I have in mind:
View attachment 158557