Off to a great start!
As an in-wall tank owner I would recommend against painting the back of the tank.
Since you dont appear to have access from the front you are going to rely on looking through the back glass on a regular basis to place frags, retrieve corals that fall off the rocks, catch fish, etc. I use a piece of black corrugated plastic from a sign shop held on with velcro and it looks pretty good. Not at pure black as paint would but at least it is removable.
A shadow box sounds like an awesome idea and would definitely be an upgrade to what I'm using while also allowing you to remove it should you need the view.
Excited to see how this turns out!