Therefore no matter how much you might be within the right, I would personally not be willing to record the police unless it was under the direction of another police force that was ready to barge in.
Present this to a lawyer and watch them work their magic.
Good luck!
https://m.huffpost.com/us/entry/us_2201016?guccounter=1&guce...
You're really trying very hard to downplay the risk of gathering this evidence. If you are discovered and you gathered evidence, you might have that evidence confiscated or you might just be dead. If you are discovered but didn't manage to get the evidence, you might be charged with wiretapping. (Or they might just kill you anyway.) Even if all works out in the end, you're looking at a legal battle. (Most people don't want to be the one fighting a case that sets precedence even if federal courts largely seem to rule in their favor.) And all that struggle is for evidence that might not even convince anyone to start an investigation and/or may not be admitted to court.
Now, could it still end badly? Sure. If they are willing to break the law for a bribe, then why not take it further if they find out you are recording.