Yeah, A few thoughts (mostly repeating what's been said already), but as someone who's currently working to turn a killer tank into a successful one I know exactly what you are going through. So here are a few comments:
1. MORE LIVE ROCK, as people said already, this is KEY for bio filtration. I'd add a bunch of "cured" rock if I was you. Also, after adding the rock, I'd monitor your water chemistry (specifically Ammonia, Nitrite, Nitrate) every day, as you may get a mini-cycle when adding new rock.
2. Once you have some more rock, and you have stable water chemistry readings you can start (slowly) adding a few hardy fish. Damsels are super hardy (and cheap), though they can be mean buggers. Perhaps try a clown fish or a damsel first. Don't rush to stock the aquarium, make sure any new fish has settled in for some time (i'd say at least a month) before adding anything else.
3. Your skimmer and powerhead situation sounds ok.
4. Make sure you are using RO/DI water for water changes and top-ups. Also, what are you feeding? Flake foods are not great, perhaps someone else has thoughts on feeding as I am not an expert here...
5. Finally, that sump sounds like it could use some renovations. Bioballs are considered by many to be nitrate (nitrite?) factories, and are generally avoided. You'd be best off turning that sump into a refugium, and ditching the bio-balls altogether. (Anyone have more experience on this to help out?).
Anyway, those are just some thoughts from someone who is going through a similar process as you.
Cheers, hope this helps.
Steve