Late to the thread, but I have a couple of thoughts.
Regarding that space in the back. I'm thinking something with some puff, but squat enough that it won't get too tall and distract from the hardscape. My first thought was blyxa and let it get a little beligerant, like it does in
this tank.
With the biodiversity.... It don't see it happening on a really appreciable scale, unfortunately. Your tank is stocked with mostly predators, who will be pretty ruthless on your copepods and various invertebrates. A tank that size would need dramatically less fish or much more space to cultivate enough invertebrates to be meaningful.
Also, black vs blue: Black by a mile.