Having extensive experience in criminal courts, I can tell you that no DA that I know of would ever attempt to prosecute a police officer. For a start, the DA relies on its close cooperation with the police as 99% of the DA's office business is brought to them by the police. Secondly, any DA will tell you that convicting a police officer is incredibly hard. Juries generally do not like to convict police. Judges even less so.
Having studied thousands of police reports, what is almost worse than testilying is that most reports are works of fiction. And the reports are what gets the defendants into court in the first place. Testilying only matters if an officer takes the stand, and an officer often only takes the stand in a trial. And 99% of criminal cases are resolved by plea before trial in the USA because a defendant does not want to risk a trial where the officer can lie and send them away for decades.
Having studied thousands of police reports, what is almost worse than testilying is that most reports are works of fiction. And the reports are what gets the defendants into court in the first place. Testilying only matters if an officer takes the stand, and an officer often only takes the stand in a trial. And 99% of criminal cases are resolved by plea before trial in the USA because a defendant does not want to risk a trial where the officer can lie and send them away for decades.