Look at the pump curve. As head pressure increases the flow decreases rapidly. Your actual head loss is much greater than 6 ft you have to include the velocity losses from the 3/4 inch pipe and minor losses from bends. Total loss is probably around 10-11ft. I would suggest a external pump. Large external pumps like reeflow or cascade have roughly same head his but much better curves. If you look at the curve for the jebao it is concave which is terrible.
Iv had jebao pumps and when it had dual 1 inch returns it produced 1350 gph max power dct15000