I've looked seriously into doing this, and there are just too many situations where I'm expected to have access to a particular app or website on my phone in day to day life. Maps are also a killer feature, and I use instagram messaging with some friends here (expats with unstable phone numbers).
The best I've had was to turn my phone greyscale, which really helped me cut back on any short form video consumption, but my endless reddit scrolling is still there.
You could get a bad smartphone. For a while the iPhone 6 Batterygate Edition really made sure I'd only use it for essential stuff, cause even typing on the keyboard was a waiting game.
I use a thing called Brick. It basically locks all the 'fun' stuff down using the phones built-in features, but you can unlock it by scanning an nfc tag thing.
I leave the brick in my car. I can still scroll if I want, but I have to intentionly get up and move to do it. Most of the time I decide it isn't worth it.
It cut back usage like 99%.