Yeah, no. There needs to be (A) pressure tank close to the pressure switch. Any additional pressure tank further down the line, past filters/etc., or higher in elevation will fill at a reduced rate due to friction loss and height. Many times the first tank fills and shuts off the pump, only to drain or partially drain to equalize pressure with the tank further down the line.
Pressure tanks are only needed to prevent pump cycling. They are not intended for supplying peak or really any demands. A 119 gallon size tank only holds 30 gallons of water anyway. Then you have to use a 20 PSI pressure switch differential to get the 30 gallons. If you have a constant pressure pump and and it maintains 50 PSI constant as suggested in the drawing, a pressure tank is completely useless. No change in pressure means no water in or out of a pressure tank. If the pump control does use a pressure differential like with a normal 40/60 pressure switch, the larger tank will be seen as additional load for the pump to supply, making pressure during peak demands even worse.
The problem is more likely the pump is not large enough to keep up with peak demands. With 50 PSI at the pump there will be 40 PSI or less on the other side of the filter/softener. At high flow rates the friction loss through the filter will be even more. With a gauge at the pump and another after the filter/softener you can see the pressure loss that needs to be made up for. If at high demand you are losing 30 PSI through the filter stuff, then the pump needs to produce 80 PSI constant instead of 50 PSI.
Constant pressure is a good thing as long as the pump is large enough and the constant pressure controller reacts fast enough not to cause lulls in pressure. But a larger tank won't help. Counterintuitively, a smaller pressure tank delivers better pressure because you don't have to wait as long for the tank to drain down to really low pressure before the pump starts.
