One of the biggest hurdles that I've experienced, and that I've seen, in switching to a dumbphone is that we use our phones for useful things, that make it harder to switch away. I've found that trying to switch first, and then move everything over to other alternatives, does not work very well. I had initially tried switching to a dumbphone around a year ago, and then when I realized I was missing out on things like my calendar and my music, I ended up trying to move those to their own 'things' (a planner and an mp3 player). Changing so many things at once, though, made this difficult, and I ultimately ended up going back to the smartphone.
Earlier this year, I finally had collected enough music where I could switch to an iPod for music, and this was while I was still using a smartphone. Around the same time, I had finally fully committed to using my planner, rather than going back and forth. Then, around a month after I had switched away from streaming and moved to the planner, I got the urge to switch back to a flip phone. This time around it was so much easier, because I had already made the other changes before trying to drastically change my habits around my phone. This meant that by switching, I genuinely felt like I was just getting rid of a distraction, rather than losing conveniences I had.
All of this is to say that I feel like the best way to switch to a dumbphone is to use your smartphone like a dumbphone first. You'll still probably end up wasting time on the internet, but if you can move the useful things that your phone does to other devices/things first, it becomes much easier to get rid of your smartphone, once you realize the only thing you use your phone for is as a phone and to waste time.
I have a sample size of 1 for this so take all of these ramblings with a grain of salt, but maybe this will help someone!