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..