The problem is not with individual cops, I know a lot of really good cops, first among them my aunt who always fought to keep the institution responsible and upright by investigating cases of police misconduct and for that she was threatened and ostracised.

The problem is that the institution politically is largely opposed to checks and regulations regarding their members. Prosecutors drop charges of police violence if any investigation takes place at all (only 2% will arrive at a charge). And if it comes to a trial, policemen form the ol' "Wall Of Silence" (also the case of Oury Jalloh)

Amnesty International says:

"Leider gibt es viele Einzelfälle, bei denen die Ermittlungen [...] unbefriedigend bleiben"

At what point do "many individual cases" stop being individual cases?

Slesvig and Lower Saxony being model police forces doesn't take away the problems with hamburgian, saxon, north rhine westphalian, hessian, baden wurtembergian and bavarian forces.

The GDP fighting fascism in the police is a good thing, but the unions resisting the establishment of independent investigators is not.

Other countries have a much more transparent handling of their cases of police violence and our german police institutions are lobbying hard to keep those regulations away in our country.

This opposition to control and regulation is a HUGE problem, especially when the people holding the states monopoly on violence are forming an esprit de corps that prevents individual good cops from blowing the whistle on bad behaviors.

I would urge you to look into the KivAPol research project: https://kviapol.uni-frankfurt.de/