I found a better picture of the output of the filter. This is when it was 3/4" pvc. It is now 2" but has the same design. It fills from the bottom and goes down the pipe you see in the picture. I get a vortex as the water is going down this pipe. I think this is where the majority of my air problems are coming from. The only thing missing is this picture is the plumbing for the inlet which goes all the way to the bottom of my barrel.
So the vertical pipe is a supply to the suction side of the pump? If it is there could be a vortex or spiral of water like in a flushed toilet that is drawing in air. If that is the case try putting a T on top and maybe a flat disk on the T if it still causes the vortex.
Sometimes all you need is a piece of wood or foam floating above the vortex to stop air being drawn in as long as it does not seal off the pipe. We used this at time when drafting water to fight fires.
Look up Durso overflow diagrams, it is basically a 90 degree elbow going on the top of the stand pipe, another small piece of 2”’pipe, then another 90 degree elbow facing the bottom of the barrel. Hole drilled in top of 90 degree elbow over the standpipe, then shove air line tubing in hole. Should only cost you a few bucks and a few minutes to put together.