For the FBI, there is such an organization. The FBI Inspector General investigates malpractice committed by FBI officers. However, the scope of their ability to fix problems in the FBI is limited because prosecutors rarely take up cases against law enforcement. They also hold FBI officers accountable by writing reports for congress. Again, the usefulness of this is limited by the unwillingness of Congress to act on this information.
(Inspectors General exist for for every federal agency)
It has to be an organization separate from any specific police agency, with prosecutors solely devoted to prosecuting police crimes, complete with their bonuses and careers linked to convictions complete with scummy plea bargaining behavior.
There also has to be new crimes of negligence for things like 'body cam turned off' or 'records suddenly missing' and so on.
(Inspectors General exist for for every federal agency)