As I paged-down to start reading, I got four words in ("Mindlessly checking social networks") before four social network badges animated in from the left. Seems a little ironic.
Regardless, it was a good read. Your idea about dumbphone+SIM-less smartphone is something I've not heard before and might try.
I was first using a dumb phone but soon realized that I need at least a maps application when I am abroad in a foreign city. Not being able to search for addresses on the go is also very unhandy if you don't wanna visit police stations around the world.
For my current Android I decided to get an upstream open source one (one out of four, to be exact, because there are not many blob free ones). Switched to OmniROM head builds without any google integrations, which kinda has the same effect as a dumb phone.
You would not believe what runs through gapps. Searching a bus? Nope, requires google. Take a note? Nope, requires google. Edit a markdown file? Nope, requires google.
f-droid and its analysis of apk files is a godsend.
My phone stays silent, with ublock0 in Firefox, and never shows a notification if I don't want it to. Use contact stars for allowance of emergency calls in DND mode.
I love it when I realize I haven't eaten at 21:00 because I could keep up the focus so much.
the absence of a digital map/route planner also has advantages in that it removes the "sterile tourist bubble" and forces one to interact with the local people.
Android user here. The addition of "Focus Mode" has completely changed my approach to my device and, by extension, my life. Various personal relationships have been noticably improved for the better as I can concentrate on them better.
I block all of the classic social notifications/apps, and I keep it on 24/7 apart from times I deliberately go out of my way to consciously press "Take a Break" for 5/10 minutes. The deliberate friction here is, of course, a welcome feature.
Regardless, it was a good read. Your idea about dumbphone+SIM-less smartphone is something I've not heard before and might try.