Conversely, if you turn off data, there are not many possibilities to spend huge amounts of time on your phone. (And yes, there are still plenty of useful offline smartphone apps.)
The good news is that you don't need to set a very high bar for a better use of this free time. Read a book. Go on a small walk around the block. Take care of your garden, if you have one. Do simple chores. Do nothing at all. Almost anything is better than smartphone usage.
That might sound obvious and borderline stupid, but it really feels like I had forgotten that for a while. This insight made cutting down my smartphone time dramatically easier and much more successful. Because while I had strategies to replace bigger chunks of social media consumption with reading books, going for walks and other hobbies, you can't really read a book if you just have a short downtime (like waiting in a queue). So I pulled my phone out to bridge that little bit of downtime which eventually always crept back into more and more situations until my screen time ended up right back where it started.
Realizing that I can just 'do nothing' made a huge difference. I also read a paper that found that boredom plays a huge role in creative thinking, which made it easier for me to do nothing because it suddenly felt like that was a productive activity.
It's a little sad that 'taking care of myself' wasn't a good enough reason to accept boredom into my life and that I was only able to do it once I believed it would have an effect on my productivity, but it worked very well and I have yet to see a downside.
I am sleeping better, I am more emotionally balanced, I feel like my problem solving skills have improved and I'm slowly, but noticeably, increasing my attention span.
All in all I can highly recommend reducing mindless doomscrolling as well as accepting more boredom into your life.
This was definitely only partially effective, probably because I already had bad habits from my laptop. OTOH, to this day I do have at least a rudimentary deeper reading/writing flow for my phone I can focus on without too much conscious effort, so it wasn't useless.
My conclusion is that most of us probably would benefit from either intentional choices or self-imposed restrictions... but most of us could also probably benefit more from a combination of both.
This sentence is a good tldr for the book, Atomic Habits.