Sounds like a fun project!
I assume based on your 18,000 gallon estimate, that your pond is roughly 20'x20' by 6' deep?
If so, then there is a limited mass of predator fish (BG and catfish) that can be supported in that volume. (Treating it as a "normal" pond.)
If you treat it as a Recirculating Aquaculture System (RAS), then obviously you can go much higher. That appears to be your plan.
There are some RAS experts on the forum. You are way out of my expertise, but I believe they all say you have to intensively manage that type of system, especially as your biomass increases.
Good luck! We look forward to some good pictures when you get everything going.
