Mistermoose,
The system that you have described is almost identical to my design which is up and running. I have two IBC Fish tanks that overflows when the pump that is in the sump returns water to the fish tank. The fish tank overflows by syphon into my settling barrel filter and that overflows into my moving bed biomedia filter and from there it overflows into my raft growbed. The raft growbed is at the same level with a sump barrel that is partially sunk into the ground. The water in the growbed moves via bridge syphons into the sump and replenishes the water that is pumped into the Fish Tanks.

I only just completed my barrel filters and put in the bio filter barrel the K1 immitation media. Although the flow rates of the whole system is now working well, I still need to refine the filtering system. My K1 doesn't circulate as I've seen on some videos. I think it needs to age an develop biofilm to become heavier, or I may need better air pump/diffusers.
Kingjam