So, after like 8 months of dumbphone only, I've given up.
It wasn't one majorly annoying thing, but just a non-stop death by a thousand cuts. Modern life really requires at least possession of one of these stupid little rectangles, and if you don't have one, you get slowly nibbled to death by the ducks of modernity.
So, rather than redouble my efforts to bend the world to dealing with me wanting to be a bit of a luddite weirdo, I've given up and just..... bought an iPhone SE and paired it with an Apple Watch 8 I already had.
See, the thing I really didn't consider is that I pretty much already had the ideal dumbphone: this AW8 is a cellular version.
It does phone calls, text messages, and has sufficient ties to modern services (music, podcasts, audiobooks, maps, etc.) that it is, by itself, a 60% solution. And just for perfect clarity: there's a lot of things wrong with the watch that make it not an ideal device, with the biggest one being really not fantastic battery life.
For everything the watch doesn't do, I also have the phone, but the phone isn't strictly required, and I can simply leave it at home when I don't want to deal with all the modern smartness and just rely on the watch.
For sure, it's not a cheap solution since an iPhone and a cellular watch is a giant investment even if you go for the "cheapest" versions, and I'm paying for two cellular plans (though, with US Mobile it's $96/year for each so, relatively speaking, still pretty cheap).
I've been in the same boat, so i got myself a cheap phone (actually a hand-me-down from my mom in the form of a Redmi 6A lol), flashed lineageos and didn't install gapps. Basically as dumb as a smartphone gets. Then you get fdroid and aurora store and you're good to go. However, i use that phone only when i need to focus, so about 5-6 hours a day, so unfortunately i still have a smartphone. So maybe it would be worth it for you to look into that sorta thing, definitely has helped me regain control.