At this point, you should take out the venturi and clean it up along with the piping between it and the tank. Cut in a draincock while you are at it with a decent sized ballvalve so that you can run more GPM tests.
You will find that you get more GPM at the turn-on pressure of 29 PSI than what you get at the turn-off pressure of 50 PSI.
On the side of that venturi, you will probably find an adjuster that controls how much water is forced through the small orifice and how much bypasses. It should be adjusted to suck air for at least two thirds of the pump cycle.
Run some GPM tests after the iron filter both with it in-line and in bypass (the black valves). That will give you some idea of its condition. You can temporarily open up the bypass on the venturi (the small brass setscrew) to allow more GPM for a manual backwash. You can lightly thump the side of the iron filter tank while backwashing to get things moving. That might bring it back from the dead. If the air remover is installed on the wrong side, there's a chance that filter media blew up into the top basket and plugged it up in which case you probably need to take the head off and clean it.
In the end, if you need more GPM than that system can deliver, you will have to forklift it out and put in a system that doesn't use a venturi to aerate the water.