I am a Stiebel Eltron heat pump owner and developer with some time to spare. In the past I experimented with the Modbus connection to the ISG, primarily to monitor the exact power consumption of my heat pump.
Unfortunately the ISG only records daily power usage, and only the total kWh is available in the Modbus registers. This limitation led me to several websites where users had modded the ISG box to add the missing Modbus registers. I modded my own ISG and developed a Homey app that uses these additional registers to access the data I needed.
The app is currently not published and only installed locally. The last couple of years the app has matured and is stable, but it needs some finishing touches like translations. If there is any interest, I am happy to finish the app and publish it to the public. Most of the capabilities of your app are available and I can add the missing ones.
