We should really have a beer. I started in advertising when I was 15. (My dad said "sell shoes or find another job"). I was in and out of agencies through my twenties. Advertising is a fundamentally bad thing. But if your clients trust you, you can sometimes use it for good. You just have to actually twist the hell out of their message to do that, use their money in ways they didn't anticipate, and always have a really concrete idea of what you want to accomplish. All while making it seem like you're just having fun.
One thing it taught me is that there's no "system" in capitalist america. Not the way people talk about it. "The system" / "the man". The system is us. Just people in offices deciding how to frame something that will reach 100 million people. I was hated by teachers in art school for arguing my case in fine art crits. But arguing for showing something they don't like is the same skill that lets me argue the minority case for putting out controversial, socially meaningful ads that might get us in trouble, instead of inoffensive corporate pap.
You can always do good in advertising, simply because you have more power than almost anyone else. The temptation, and the easy thing, and the thing that pays, is to perpetuate little evils. But it's surprisingly easy to do good. It ain't "art". It's arguably more important, though. You're in the trenches where the battle for minds takes place.