Hey there, thanks for the input!
Your layout idea makes sense! I think I will raise the second hill higher. It looks like I won't be planting dwarf hairgrass in the front anyway, so the substrate doesn't need to be so deep. Unfortunately, this is the only sand I have left to work with, so I can only sift it around.
As for widening the valley: design is difference.
At least when it comes to layout/environment. I'm keeping in mind the 1/3 rule, and was actually considering moving the middle rocks more to the left to make the valley even smaller.