Achieving a balanced Iwagumi scape is always a challenge. But the height/width ratio of your tank makes it even harder than it would be in a wide, shallower tank. In any case, the volume of rocks you have is insufficient given the height and total volume of your tank. You can consider adding two large rocks placed according to the golden ratio. One option is to have the tip of the larger rock at ~2/3 of height the tank. Other option is to go really big and have the tip of the larger rock breaching the water line, which would probably look nice in your open tank. Note that the largest rock you have at the moment does not even reach half of the tank's height. In this process you will also need to raise part of substrate significantly and stabilize it. Consider adding lava gravel to the bottom and maybe a plastic egg crate structure to help raising the rocks and to save soil/substrate. A couple of scapes below in tall tanks from EALPC and Green Aqua.
As an alternative, you can try a "minimalistic" Iwagumi scape with a lot of empty space. But those are even harder to balance and the examples are mostly found in shallow tanks, not in tall tanks.
Have fun!
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