Plumbing connection to cistern

Connecting metal to plastic is a recipe for a leak. Wrap the threads with blue monster Teflon and then put pipe dope on top of that. Go plastic to plastic if you can.

If you're really concerned and don't care if it ever comes back apart, some 5200 marine adhesive sealant on the threads will absolutely keep it leak free forever. It will not ever come apart without breaking something.

That's hardly necessary though, and 5200 is expensive stuff. It isn't going to have any pressure on it. I'd use regular pipe dope or teflon tape and roll with it.

The only thing I'd double check is that the bronze bushing might well have BSPP threads. A tapered pipe thread will screw into it but it won't seal easily and if it does, something like 5200 would be necessary to get a reliable seal.

It will be about 10 feet below the ground surface, so I'm hoping not to have to ever dig it up. I'm just looking for the best way to go from a 2" PVC female threads to the 1" polypropylene line.

All of the plastic hose barbs I've ran across seem like they are made for sprinkler systems and would be a weak point. I think I will go with a Sch 80 PVC nipple to a brass reducer (brass female to PVC male threads) down to the bronze barbed insert. I will use the marine 5200 for sealing the brass reducer to the PVC nipple. That way if it ever has to be replaced, I can take the entire assembly out at the PVC nipple.