All I have is a premise, the rest won’t be hard for a good screenwriter…
A 20-something, hard-working, caring, but poor, inherits/wins enough money to not have to work any more.
They have been upset by the level of crime, homelessness and drugs in their area.
They decide, now with time on their hands to do something the police don’t have the resources for – surveil people who look dodgy.
It is easy, retrofit a van, or rent an appropriate apartment (or both) and watch the homeless folk to see who brings them drugs. And then follow them to see who supplies them. And so on.
Of course our hero is smart and cautious, so they only follow people for short instances, and triangulate things.
When they have substantial evidence, they go to the cops, but nothing gets done (cops are complicit??).
So they work how to destroy the network. It might be by making them all think the others are stealing from them. It might be by physically destroying them. It might be outing them on social media. Or all, a blitzkrieg.
The key is that it is incredibly easy to simply follow people, if you choose.
If you want to make it all Taxi Driver, get them following someone because they fancy them as well.