There is no reason why the one well pump cannot do all the jobs required. That is kind of the point of "constant pressure". "Constant pressure" really means "variable flow". In order to maintain "constant pressure" the pump system must be able to "vary the flow" to match the amount of water being used. That means the pump should be able to vary the flow to match a single 2.5 GPM shower in the house, as well as a 6 GPM sprinkler in the yard, or both at the same time.
There are new style VFD's available but, VFD's are not new. Pump and VFD companies keep coming out with new models of VFD's, that are supposed to solve all the problems of the last VFD. They will have a few "upgrades" for the new VFD to work out more bugs, before they come out with the "newest" VFD that for sure this time will solve all the problems. The customer keeps being told "we have the problem fixed for sure now, just purchase the new version". A VFD is made with the same microprocessors that are used in lap top computers. Therefore you will have the same reliability and length of service from your water system, that you have with your lap top computer. What is the oldest computer you have that still works? And if you had problems with a computer over 3 years old, could you get parts for it? NO, of course not. They no longer make the Pentium 2 chip, now they are making the Pentium 4 or Centurion chip. So no parts are ever available to repair a VFD. Any problem, and you must purchase a new unit. Does this sound like a predictable, familiar, and money making proposition for the manufacturer? Of course it does. Making money for the manufacturer at the expense of the end user, is what VFD's are all about.
Cycle Stop Valves were invented to replace Variable Frequency Drives. We have been successfully replacing VFD's with CSV's for more than 15 years. When you replace a VFD with a CSV, you get the same or even better "enjoyable constant pressure". However, the CSV is a permanent fix for the VFD. No more "patches" or "upgrades". No more having to go to the well house and unplug and restart the computer regularly. No more lights with dimer switches flickering. No more interference with radios, TV's, telephones, etc., etc..
The Grundfos SQE is converted to an SQ, when the CU301 controller is removed. The SQ controlled by a CSV and regular pressure switch is much more dependable than when used with the computerized CU301 controller. Then when the little 10,600 RPM SQ pumps fails, you already have the CSV controls needed to replace the pump with any standard 3450 RPM pump, which is also much more dependable and long lasting.
Attached is a drawing of how to change out the CU301 controller and install a CSV. Also there is a before and after picture of a recent SQE to CSV conversion. Let me know if you have further questions.