Prosecutors rely heavily on police testimony during trials and their conviction ratio is highly important for their careers especially for elected ones, so it can easily create a conflict of interest.

They go after the police and somehow police officers stop attending trials or claim they don’t remember during their testimony.

There needs to be either a completely separated body from the state/district attorneys or it needs to be federal.

I was thinking a specialized police prosecution unit, on the state level. The career prosecutors there would make their careers by convicting cops.

That’s what internal affairs does, it still isn’t enough usually because it pretty much means you are stuck there once you join since your other career prospects disappear.

I think there's more risk of corruption or favouritism with IA than if it was a federal body completely disconnected from the department

Yep, in general this is what the DoJ supposed to also be doing, it just doesn’t have a mandate that can be applied to all cases.

Historically the answer is parochial corruption and interests where they don't want the problem solved regardless of what the law says. The federal level has "fifty" eyes on it while a state has mostly its residents watching it. Which is why federal hate crimes were devised.

To me a federal organization is a single point of failure. Once it goes bad, it's all bad.

In my world, the residents of a state should be the ones to keep an eye on this. They'd have a chance to affect it. Changing anything in Washington is rarely humanly possible.