The closest thing to training I've found is this: https://datadetoxkit.org/en/wellbeing/essentials/. But what worked best for me was paying more attention:
- First, I'm now aware that phone overuse is an issue. This wasn't obvious a year ago, even though I used it even more.
- Analyze my usage patterns. For example, I'd get a message on some app, open it, and end up doomscrolling. It helped me to change my notification settings so that I only get emails. That way, I can check it on the computer later, where I usually waste less time.
- I agree that gamifying is a good idea. It's precisely the main feature of the app I'm working on. Not only that, but building a "community": I started to talk about this problem with friends and learned from them. It also makes me more accountable; my girlfriend will nudge me to leave the phone alone if I'm stuck, for example.