I pretty much did what scubafish did.
First thing I upgraded was to the intank media tray, since the stock sponge one didn't do much as far as flow. Highly recommend since now, the water flow from the intake, up to the baffles and then back down again so I can layer me media. I use filter floss, carbon/chemiclean, and then finally some ceramic media for bio filtration. Kinda expensive at $50 per media box.
I also had an extra hang on back breeder box and made myself a hangon refugium with an ebay grow light.
I also bought the icecap k2 nano for my protein skimmer.
I sold my light that came with the fluval flex and bought 2 ai prime 16hd.
Im going to keep my pump since I put on 2 jebao slw20. They are super powerful, im only on 20% power. I decided to get 2 since I wanted the swaying motion for my torches that I currently have in.