Maybe you need to know that dumbphones aren’t so dumb as you might think they are.
A dumb phone is only dumb, if you never connect it to the internet. If you do, it’s another closed OS.
A lot of the Nokia dumb phones connect to Google out of the box: https://www.reddit.com/r/dumbphones/comments/oldzcl/nokia_6300_4g_and_unwanted_google_integration/
Therefore if you never connect to the internet, you can use ANY phone. If you want to connect to the internet I would recommend using something with a custom ROM.