First of all you want to use a standpipe to extend the drain above the bottom of the tank. The top of the drain standpipe needs to high enough so you don's drain the tank in the event of a power failure.
Check out Durso standpipes
Second, you need a better pump. How many inches is it from the pump intake to the where the water is being returned to the water?
That is the head height. A pumps output is reduced for every foot it needs to pump vertically (horizontally too, but we don't have to worry about here).
You need to match the pump to the drain!
There should not be a significant difference between pvc
and flex in how much water can pass through the diamter of the pipe.