the car loses power when moving uphill. Problem seems to disappear with a full tank of fuel. 1/2 tank and you’ll be on the shoulder, creeping along with flashers blinking. Mech has changed filters, no water in fuel, transmission seems fine, RPMs are normal. Flat ground doesn’t show issues and all seems fine, even with loaded vehicle. At a loss regarding both vehicle power and understanding.
The next time this happens, try removing the gas cap and reinstalling it. If you get a ''whoosh" and the problem goes away, that’s a good indication that your vent line is plugged or your charcoal bed saturated and the gas tank cannot breath in. That can cause a vacuum to develop in the tank as the pump pumps out gas. The vacuum can prevent the pump from being able to continue pumping. It can also cause premature pump failure.
Post back with the results.
VW’s had troubles with the EGR system becoming clogged.
Have an independent mechanic clean the EGR system.
A clogged EGR will not reduce power output of the engine. In fact, it can often increase power.
It’s unlikely that the fuel tank is drawing a vacuum either. The gas cap has a check valve inside it that allows air to enter the tank but not escape.
All your symptoms point towards a fuel delivery problem. Perhaps the pick-up screen inside the fuel tank is clogged. Try a different mechanic. Preferably one that is familiar with VWs.