Well first take care to know what you're getting into. Cops who rat out other cops get death threats because the police are a mafia.

I'd get in touch with a whistleblower organization of your choice and ask for advise, but the basics would be wear a tiny hidden camera and try to record training and anything illegal. In most states quotas, for instance, are illegal per se, but it's not enforced because the cops would have to arrest themselves. A number of departments have been brought down because people simply recorded their 'captains' saying "hey, I need you to be finding more ways to fine people"

Another approach would be to show what the cops thought (if they were the mythical 'good' cop) and record them saying "Damn the mayor won't let us arrest these blm bastards who burn loot murder, but tell us to arrest maga hat wearing people; it's all political" I'm sure that kind of conversation happens every single day amongst the 5% of not shitty cops.

I view every single law figure, and alphabet agency as an immediate threat to my gun rights.

That's because they are. Every one of those cops would happily enforce gun confiscation orders and go home satisfied. There's nothing else most of them can do; if they're no longer cops their next best job is mall security. You give power hungry twats with a chip on their shoulder 20-30k more per year than they otherwise could earn in a market and think that they're ever going to risk that paycheck? nope.