You say that it will fill and drain...but when it's in the middle of doing that, does it just sit at idle for the time period that it should be running / Circulating? or does it immediately go to draining?

A. if it's draining immediately, there is some sort of electrical issue vs mechanical. Could be a faulty fill switch or flood sensor.

B. if it fills up and then just sits like it's waiting for something to do, it could be a bad board or a bad motor. My bet is that it's a bad motor. Good news is that this is common and a fairly easy fix with you 20 yr background. Best way to find out is if you get voltage at the motor ( larger of the two in the bottom of the unit. I'd recommend removing the entire unit to a garage and have it ready to test with a hose and hot wired up., can then test of 120 vts at the motor. If you don't have voltage there, then you have a bad control board. Usually the motor will hum/ buzz or sometimes do nothing, but you can feel it trying to engage in most cases. Voltage tells the whole truth though.