> I am just wondering when power starts to become misused from the legal point of view
That is indeed a good question, and one of the reasons why e.g. Amnesty has been telling Germany the second time in a row that we need an external (to the police) oversight of the police, because police oversight by the police hasn't been working very well, since they simply always say it was necessary force given the circumstances.
> You mentioned that they increased the sentence to 3 years. Sure, when?
The sentence was not increased, the bar for the "very serious case" sentence was lowered. E.g. one part of StGB 113 read before:
* Whoever carries a weapon or dangerous item with the intent to use it against a police offer.
The last part was simply removed in 2017 [0]. It now reads as:
* Whoever carries a weapon or dangerous item.
Nowhere is defined what a "dangerous item" might be, so essentially, in the current legal state, if you carry an Umbrella to a demonstration that could be a problem.
[0] (German, sorry): https://www.buzer.de/gesetz/6165/al61066-0.htm
Edit: It should also be mentioned that these cases are most often parole cases. This does not change the fact, though, that having been sentenced with such an offense will inevitably have a very serious impact on your life and employability, and lowering the bar for being sentenced with such an offense can have a serious chilling effect on people being willing to participate in political demonstrations, which I personally believe are an important part of democracy.