When it comes down to it, think of your rock as your main source of biological filtration. Typically we have more rock in our display than in our sumps, in your case, behind a false wall. Those are what you really want seeded as more so than not they will always be in the tank.
I personally like filter socks, skimmers, filter pads and a sponge. Some would say sponges are more of a nutrient trap (nitrate factory) just like the bio balls used back in the day. Some swear by the ceramic bio balls or bio plates (theres quite a few options now on the market) In theory they are great to have if you have minimal live rock. I had them initially in my sump but removed them. I saw no difference.
Filter floss or pads are great. They are cheap and you can just toss and replace. Sponges will need to be rinsed out as they will hold a ton of detritus. Most people shy away from sponges as its just another thing to clean .
If you chose to do the ceramic bio balls just seeding the tank with bottled bacteria will be good, along with time. It takes time for a tank to really establish. This is going beyond the initial nitrogen cycle.
Hope some of this helps. Good luck man!