I am at my wits end.

Not sure what your using for a pump and pick up. I had the same problem 20 plus years ago. I was using a milodon pan with a swinging pick up. The type where you have to put their longer gears in your pump and use their pick up. I used a new high volume pump. Zero oil pressure when priming. Dropped the gears in a standard pump housing All is well. There is a few thousandths difference between standard and hi-vol housing.