You have already fallen for the variable speed pump hype once, don't fall for it again! The Monodrive or any of the other variable speed pump systems available are no better than the one you have. As a matter of fact, even though they have lots of problems and don't last very long, I have found that the Grundfos has been the best of all of them. When they say that variable speed pumps save energy, they are lying. When they say that variable speed pumps last longer, they are lying. When they tell you that a new style of variable speed pump will solve all the problems that were seen in the last model, they are lying. The pump guy probably actually believes this stuff himself, because he has been lied to by the manufacturers.
The VFD or variable speed pumps are what manufacturers call a "fluid product". A "fluid product" has nothing to do with water. A "fluid product" is one that cost a lot up front, doesn't last very long, is not repairable, and must be replaced with a new one. This keeps the cash flow "fluid" for the manufacturers. The fact that it is an important item that provides water, and in your case heat and cool for the family, means you can't be without it, and must replace it quickly. Hopefully you need a replacement so quickly that you don't have time to research alternatives.
The CSV or Cycle Stop Valve was designed to, and has been dependably replacing VFD pump systems for almost 16 years now. The CSV gives the same energy efficiency as VFD, and the CSV actually does make the pump system last longer while still able to use a much smaller tank. See the attached picture of one of the most recent Grundfos SQE systems that was changed out to a CSV system. If your pump would still run, we could have just replaced the CU301 controller and used a CSV. Since your pump is also fried, you should replace it with a more dependable, and less expensive, standard 4" or 3" pump. I prefer a tank that is a little larger than the 2 gallon tank you have, but if the bladder isn't already busted, it will still work fine. Install a CSV and a standard pressure switch, then you will have a dependable and long lasting system.
I have heard countless times that using a heat pump to save energy is a moot point if the pump system does not last. Replacing an expensive pump system every couple of years cost much more than a heat pump could ever save. The real savings comes when your pump system last 20 years, and that is what the CSV is designed to help happen.
Just do a search on this forum for "CU301" and you will see how several other people have solved this problem.