A WX202 only holds 5 gallons of water. With a 2.5 GPM shower head, the water pressure continues to decrease from 60 to 40 for two minutes, as the tank is emptied. Then the pump comes on and the pressure increases from 40 to 60, as the 5 gallons is put back into the tank, at the same time the shower is still on. If your shower last longer than about 3 minutes, this process will repeat, which causes the pressure in the shower to drop from 60 to 40 again.
This is why I like a smaller tank and a Cycle Stop Valve. With the smaller tank, (WX102) the 1 gallon in the tank is used up before you get the temperature adjusted in the shower. Then the pump comes on and the CSV keeps the pump running, and the pressure steady at 50 PSI, for as long as you are using water. A constant 50 PSI will be much better pressure in the shower than an average of 50 PSI, that you are seeing now, as the pump continually cycles between 40 and 60.
Just adding a CSV to the system you have now will help. It will still hold the pressure steady after the pump is started. You just have decreasing pressure in the shower for about two minutes until the pump starts, because of the larger tank.