OK, you brought up the VFD thing, so I am sure everybody is popping popcorn, waiting for the show to start. The reason is that Cycle Stop Valves or CSV’s were designed to replace Variable Frequency Drives or VFD’s. The CSV and VFD both achieve the same goal.
The VFD is a computerized, expensive, problematic device that varies the speed of the pump/motor to reduce the amount of water from the pump, to match the amount of water being used. Many pump companies are pushing these devices because they are expensive, short-lived, hard on the pump/motor, and therefore very profitable. Which of course profit for them means extra cost to you.
A standard pressure tank system requires a large pressure tank and still causes the pump to cycle on and off a lot. Cycling causes pressure fluctuations in the house and shortens the life of the pump, motor, control box, pressure switch, bladder in the tank, check valve, and anything else in the system.
The CSV is a simple valve that varies the output of the pump to match the amount of water being used. Very few pump companies will even mention a CSV, as it is inexpensive, and makes pumps last several times longer than normal. The CSV holds a constant pressure in the house and works with a very small pressure tank.
My rule of thumb is 5 GPM per bathroom. So your 19 GPM estimate is pretty close. Another benefit of the CSV is that it will allow you to install as large a pump as you think you may need, and the CSV will make it work all the way down to 1 GPM when needed.
Even though your well only produces 4.5 GPM, the 300’ of water in the well is like having a 450 gallon tank that will let you draw as much GPM as you want. I would probably use about a 18 or 20 GPM, 2 HP pump, set at about 250’, controlled by a CSV1A Cycle Stop Valve, and a 20 gallon size pressure tank. The only reason I would use a tank that large is because I would set the CSV to run at 70 PSI with a 60/80 pressure switch because of the three story height, instead of the normal CSV at 50 with a 40/60 switch. 40/60 could use the 4.5 gallon size tank, but higher pressures like 60/80 needs a larger tank.
See this link and click on the 1 to 6 different uses in the house to see how it works.