The state of New South Wales (NSW), Australia has a Law Enforcement Conduct Commission (LECC) [1], which is a body legally separate from NSW's law enforcement agencies–NSW Police Force, which is the main state police force; NSW Crime Commission, which specialises in organised crime and other serious crime–and which is tasked with investigating law enforcement misconduct. (In Australia, there is no local law enforcement, only state/territorial/federal.) The LECC doesn't prosecute itself, but can investigate cases and refer them to state prosecutors for prosecution. It isn't perfect, but it is better than nothing. Maybe the US could copy the same idea?
(Also, separately from the LECC, there is also an Inspector of the LECC, whose job is to investigate misconduct by the LECC themselves.)