I found the best dumb iphone to be an apple watch. I can't switch entirely to something like an old school Nokia because of imessage, emails, facetime, etc.
if you want imesaages, emails, facetime etc you want a smart phone :)
The problem I had with Apple Watch was there were still apps I would need. For example, Uber doesn't have an app for the Apple Watch anymore.
It's not a full replacement, but it allows you to leave your phone at your desk or around the house for 95% of things