Your pressure relief valve is the most dependable part of your pump system. It will pop off when the VFD fails to shut off the pump. The PRV will protect your pump from a deadhead condition, which will burn up your pump. Nothing will protect your pump from having to start for every drop of water used. The 2 gallon tanks hold about 1/4 gallon of water, which can't even be used unless the pressure drops before the pump starts, and the pressure climbs 20 PSI before the pump is shut off. Usually these little VFD systems do not have a pressure bandwidth like that, so the pump has to start for every drip out of the faucet.
I watched the video
http://residential.goulds.com/constant-pressure/
And while it makes the variable speed system sound inviting, it is incorrect about several things. First they claim the VFD system will eliminate water hammer when a faucet is suddenly closed, and it CANNOT do that. They also say the pump has less stress when it gradually ramps up the speed, which is not correct. The thrust bearing in a submersible motor needs to be up to at least 50% speed in less than 1 second, or it will not get the lubrication it needs and will quickly be destroyed. And they fail to mention that the RF interference produced by those units can cause problems with other electrical devices in and around the house.
But since you fell for the hype, you are now stuck with a 3 phase motor, and cannot easily change to a more reliable system. As long as you have the 3 phase motor down the well you are stuck using a VFD to convert your normal single phase house power into 3 phase that the motor needs. That way they usually get to sell you 2 or 3 of those expensive controllers before you starts looking for a reliable alternative and switch back to a normal single phase motor.
I did find the video interesting when it discussed all the problems with the old "traditional pressure tank only system". These problems are true, and is the reason these companies are trying there best to find an alternative to the old pressure tank/pressure switch system. But they want to find an alternative that cost a lot and doesn't last very long, which is where the VFD like the Aquavar comes in.
What they don't want you to know is that you could have added a simple, inexpensive Cycle Stop Valve to the standard pressure tank/pressure switch system and solved all your problems. The CSV would have worked with a regular single phase motor. It would deliver the same constant pressure as the Aquavar VFD. But the CSV would not have cost much, and would have made you pump system last several times longer than normal, and they don't like that.
If the Aquavar has a connection for a switch to start and stop the system, you can use a regular 40/60 pressure switch, which will shut the pump off on high pressure (60) like you are asking. And then the switch would start the pump again at 40 PSI when needed. But if the Aquavar runs on a pressure transducer as I am sure it does, you would have to use a relay for the switch to be able to shut power off to the Aquavar.
It is probably best to just leave it the way it is and see how long it last. In the meantime you can be saving money to replace the three phase motor and researching some more reliable ways of getting the constant pressure you want, which is a good thing as long as you don't use a variable speed type unit to get it.