If your running a 20/40 or even a 30/50 pressure switch and you lose 20 PSI through all those filters, you have no pressure left. You need a pump that can do 60/80 pressure if you have enough filters to lose 20 PSI.
Another way to increase the pressure is to use a constant pressure valve like a Cycle Stop Valve. The CSV works with the same 60/80 pressure switch. But instead of going on at 60 and off at 80 all the time you are using water, the CSV will hold a strong constant 70 PSI with any faucet on. Then if you lose 20 PSI through the filters you still get 50 PSI to the shower.