I am starting to do that but I still find that will be annoying to keep up. The dumb device will need to be kept up-to-date because that's important for e.g banking apps so that automatically excludes "old" devices (usually 3+ years old) because most stop getting updates that fast. Even LineageOS isn't an option because it does not pass Google's integrity checks so I am afraid I will need to buy a new dumb phone every couple of years..
On my to-do list is to join a local credit union after checking whether all services are available in person. All this mess is not worth simply being able to do stuff from my phone.
The trick is to never, ever install the banking app on your phone. If you only use their web app from the beginning (account creation) and say you don't have a duopoly phone, the app will not be required.
