I've done something similar to this. I recently was in Cuba, where to get internet you had to go to public wifi places. In between that, my phone was much dumber, but very useful: you could take notes, pictures, set timers, etc.
So, back, home, I've done the following:
1. Disabled my phone from home wifi (I have 12 GB data, so this is no issue)
2. Turned off cell service to any distracting apps. Turned off notifications too
3. Set all those on a separate "internet" page
4. Went into parental controls and locked changes to cell data settings
5. Removed Safari, added firefox focus and the google app
I've tried locking down my phone before, and it never worked. Occasionally I do want to post something to instagram or check snapchat, and the only good way to do that is on the phone. So I'd end up redownloading them.
Now, I still can use them. But only if I go to a cafe or a library with wifi. So I make a list of things I want to do on those apps, and update them when I visit.
Meanwhile, I can use my phone for lots of others things. But all of them are self-terminating. I removed everything open ended: browser, youtube, etc. Those tended to be black holes.
I also found my phone would get cluttered with old browser tabs, and these would be more distracting than on desktop. So, by switching to single tab browsers, I can avoid this.
What happens if I need to google something on my phone, immediately? Well, usually this isn't true, and so I write it on a "to google" list, or walk to a computer if I'm in my office.
But, if it's truly urgent, I just go into restrictions, and talk to myself out loud about what I am doing. e.g. "I am changing restrictions, and turning on data for firefox focus. I will search 'how long to boil eggs' and then turn off data to firefox and turn back on data restrictions"
I find this latter technique helps me stick to doing only the thing I intended to do.
I've been doing this for a week since I got back from my trip, and have had no slippage yet. I'm still easily connected at my office.
I have noticed that I am checking wikepedia articles more often, and also using podcasts more. I think this is a good thing, though I'll have to re-evaluate over the long run. I may want to instead add wikipedia topics to a "research" folder so I can do them at a later date; it could be just another form of distraction. But most usage comes while reading books, so I think it's fine.