Change your Stiebel Eltron for a high end unit with the digital display
I have one, allows me to set the temperature on the digital display.. It will increease power dynamically and regardless of the external water temperature or how how many Liters/minute of flow I want. You can set it to give you a "drip of 2.5L/minute at say 34c".
It also gives you all nice stats as to how much water you are using per year etc. Has preset temperatures for filling the bath, how many liters. Preferred shower temperature. It cost 6K but it is well worth it.
I tortured myself for years and wasted hours per month with a horrible unit like you have.