Remove the lower panel and in the middle will the be while sump clean out. Have towels handy because any water inside the sump will quickly be in your lap. Open the clean out and see if it is full of 'stuff' that may be slowing the flow of the drain water.

It sounds like water may be remaining the in the sump area, and if you are hearing cavitation sounds, and the sump is clean, I would pull the pump off the side of the clean out and see if the impeller is loose on the motor shaft, or even has a broken fin or two. The result will still be to replace the pump, but at least you would know what was going on.

You may want to do the pressure switch tests to verify it is working OK. The amount of water is monitored by the pressure switch as set by the control board. You mentioned the pump sounds like it needs to be primed, and I am assuming this is while there is still water in the tub. I am wondering the reason for the noise if there is water to pump. If the pump clean out is not blocking the flow of water, is there anything possibly in the sump (the rubber hose connecting the tub to the pump) that may be slowing the flow? A larger load is going to use more water than an small load so it may not be able to drain quick enough. If you are running the diagnostics and it won't go past the drain test, I would again look into the operation of the pressure switch, and then the control board. But verify there is absolutely noting that is blocking the flow of water. You wouldn't believe the things I have pulled from sumps.