I agree with the thought on sand. I find it tends to just get scattered as I do most anything in the tank. But then my tanks are often being "stirred" so maybe yours is more stable. Though I don't think of college as stable for many things!
I might think of really thin slate flakes kind of nestled down into the other.
I agree that some taller/bushy might look good. I like to give more height as my fish tend to like to get in under some of the leaves.
Maybe some of the small/mid size swords? Red myrio can be nice and can be cut and replanted to bulk it up when it gets tall. Needs good light to be red. Depending on how the tank runs, it might need trimming more than best. Six stems of simple hornwart planted together in the left rear corner?