Regardless, it was a good read. Your idea about dumbphone+SIM-less smartphone is something I've not heard before and might try.
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.
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.