There are a few nice hints in the comments already, but I'd like to remind one feature that may actually be "the dumbphone" experience but more modern.
Ultra power saving mode on Android.
It may differ from vendor to vendor, but in my experience it works exactly like that (been able to use that in old Samsung, Xiaomi, Realme, Moto). It changes screen to greymode, simplifies launcher by a lot, cuts notifications, still allows to use a selection of apps, including gps.
It customizable - you can keep some color, choose which apps can be accessed (on my current phone up to 6), make calls, text or IM. As a bonus it takes a few days to drain the battery.
I use olauncher which has a super minimalist home screen. It doesn't block apps but it doesn't use icons. Instead you type the name of the app you want which I find makes me more purposeful with my phone usage. Turning off notifications for virtually every app also helps. If it's truly important I'll check it. Otherwise it's something I can look at once a day/week/month depending on the app.
I like this approach but it was a bit much for me. Especially no icons - a lot of apps do those automated translations which results in weird names. I have several ones that are called the same due this reason. It annoys me that I can't find them.
Went with Niagara launcher. It is somewhat limiting sometimes, but truth is that I could setup most important things easily. It doesn't really let you do multiscreen homepages so I stopped using many apps. And I am grateful for that.
All these hacks remind me of what I tried to do to cut down smoking cigarettes. They work for a bit but doesn't solve the real problem. The only thing that really works long term is to simply quit smoking cigarettes completely.
Ultra power saving mode on Android.
It may differ from vendor to vendor, but in my experience it works exactly like that (been able to use that in old Samsung, Xiaomi, Realme, Moto). It changes screen to greymode, simplifies launcher by a lot, cuts notifications, still allows to use a selection of apps, including gps.
It customizable - you can keep some color, choose which apps can be accessed (on my current phone up to 6), make calls, text or IM. As a bonus it takes a few days to drain the battery.