Just one person's opinion, but you did ask
Try for at least 2, preferably 3 islands of varying height, the tallest not more than half the tank height. Negative space has a great visual effect, helps with flow, and gives fish some room to fly around the tank. And lastly, the islands give you three sides for coral placement versus the rock wall only giving you the "face" to work with.
"Un-clutter" the sandbed and fasten those corals to your islands. Also personally, I am not a fan of scallop shells; they don't look natural.
I don't know how you manage your sand, but if it is holding a lot of detritus, I would run carbon and do this work in stages. If you end up having to reduce your rock volume, I'd also suggest stuffing as much of it as you can into the sump to maintain the same level of biological filtration. Just minimize any potential shock to your system.
Good luck and have fun.