I am afraid you just jumped from the frying pan into the fire. Those standard submersible pumps are much more dependable and long lasting than any variable speed pump system. You just have to make sure standard pumps are not cycling on and off continuously. A Cycle Stop Valve eliminates cycling, which will triple or quadruple the life of standard pump systems.
The Subdrive uses a pressure switch with only 1 PSI between on and off. This causes the pump to cycles on and off, or "ramp up and down" as variable speed people like to say, many times more than a standard pump system. When running your heat pump or any other low flow rate, this switch will cycle the pump 45 times per minute. If the pump runs 24/7, that would be about 2 million cycles per month. With this amount of cycling the switch is usually the first thing to go but, no part of the pump system can withstand 2 million cycles per month for very long. This also means the check valves are opening and closing 2 million times per month. So multiple check valves just means multiple check valve failures.
When the heat pump or no other water is being used, the 1 PSI pressure switch only allows the use of about a teaspoon of water from any size pressure tank. This means a small leak will cause the pump to cycle multiple times when no water is actually being used. Either all of the check valves have already failed, or you have a small leak somewhere else. Check the overflow tubes in the back of your toilets. Look for a dripping faucet. Even your electric valve on the heat pump could be leaking a little.
Just turn off power to the pump while no water is being used. You will see the pressure continue to drop until you find and fix the leak. There are always leaks in a water system, as there are only two kinds of valves ever made. There are valves that leak, and those that will soon leak. Toilet float valves, faucets, electric valves, even check valves will all eventually leak. You just have to design a system that is not destroyed by small leaks and the subsequent cycling of the pump.
You can convert a Subdrive to a more standard type pump system by using a Cycle Stop Valve. The enclosed picture of a Pside-Kick complete pump control kit, comes with a plumbing and wiring diagram to convert the Subdrive to a Cycle Stop Valve system. The Subdrive switch is replaced with a standard 40/60 pressure switch. This gives you a full 1 gallon of usable water from a still small 4.4 gallon size tank. This 1 gallon of water from the tank can supply a dripping faucet or a weeping valve for quite a while before the pump must start. Then the CSV set at 50 PSI, will eliminate any cycling while you are using more than 1 GPM anywhere in the system.
Your only week link in the system will then be the Subdrive electronic controller and problems it causes to the motor. The CSV will make them last longer, and when they do fail, you will already have everything you need to control a standard pump system. A standard pump system with a regular single phase motor and standard control box will last many times longer when controlled with a CSV.
You won't be able to get rid of the Subdrive controller until you switch back to a regular single phase motor. And you won't be able to switch back to a standard single phase motor until you get rid of the Subdrive controller. And you won't be able to get a pump system to last long enough to get your money's worth, until it is controlled by a Cycle Stop Valve.