Don't keep the bunches of plants all in a bunch. Separate them and plant them individually, about an inch apart, so the light and circulating water can reach all of them.
My personal preference is to avoid symmetry, so I find your layout much too symmetrical. It does look like it will look good though.
You won't be able to keep the two substrate materials separated. Over time they will mix and blur the dividing line between them, unless you spend a lot of effort trying to maintain the border between them. Under water, the individual particles weigh very little, so water currents and fish easily mix them together.