How is everything running without the supercharger? You didn’t give any information about the propeller setup. That is critical to understanding your RPM issue.
The purpose of a supercharger on an aircraft engine is to allow it to maintain adequate MAP at higher altitudes. Are you seeing this work?
There are many parameters that may not work in the sim or at least it’s not known how they work. If you can find an example of another plane that implements this parameter, you may need to analyze the parameters it is using as a guide.