Glad my responses are helping you think through this.
Eager to hear what others have to say.
One of the greatest things about this hobby is that once you have the hardware you can try something and - if you don't like how it turns out - you have another day of playing in the water as you tear it down and try something new.
I'm not sure about the specifics of placement, proportion, etc. But I think that some who knows more than me might suggest that your current placement is inconsistent with the "golden ratio." As I look at it, the current focal point is somewhere between the 2 largest pieces of wood, about 1/3 of the way in from the R side. This is emphasized by the fact that you have the 2 pieces angling away from each other.
Unless you are trying to do something daring like have the focal point be negative space - like a valley, it strikes me as a little confusing to have the 2 really nice looking pieces positioned in a way that my eye doesn't know what to concentrate on. If you want that "valley" to be the focal point, you might want to stick some bright red plant in the back right between the 2 pieces. Just a thought.
One easy thing to try might be to turn both pieces around 180, so they flow towards each other and draw the eye to the point between them - if that makes any sense. Another thought is to turn one or the other 180, so they both tend to flow in the same direction.
You can accomplish all of your goals. The easy way is to use the smaller pieces to grow plants on, and pile them around the bigger pieces to create your caves. Then leave the most interesting parts of the bigger pieces bare - tho you can selectively attach a few smaller plants higher up to emphasize that dimension.
Final thought, until someone else chimes in - think about emphasizing the wood from rear to front. If you plant the back parts and have bare parts coming out towards the front glass, it can give the impression of depth.
Heck, if I go on like this I might almost convince myself that I know something about what I'm talking about! Suffice it to say I most definitely can talk the talk better than I walk the walk!