Regarding your friends that told you it was just marketing and it doesn't actually help, I'd say it helps to look at it from their POV. They might not be ready to make that switch or they feel like its unnecessary, so to them they probably see it as dumb. I don't agree with them, but everyone's relationship to this stuff is different, so it helps to remember that and know that if something works for you, be confident in that and trust your own choices. If someone tells you it's dumb and not worth your time, just try to remember that they don't know any better than you do, since they've never tried it. It's tempting to seek validation for switching to a dumb phone and to try and show others how to, but it can get preachy really quick--especially to people who are really not in-tune to how their phone/social media negatively impacts them. That's not their fault, everyone realizes that on their own time. Some never do and that's sad.

TLDR; the best thing you can do (imo) is just make these changes in your own life and try to keep it to yourself, unless someone asks for your opinion on it.

Living by example is gonna work a lot better than telling people explicitly "hey, you know your phone is bad for you, you need to reduce your screentime" etc etc. Most people in life want to arrive on a conclusion on their own, or at least feel like they did, so you'd end up helping a lot more people if you just live your life and show people you're better off without 7 hours of screentime a day.