You simply should not use a smartphone without security updates. It's as good as a brick in this case. (Or maybe never use Internet and Bluetooth on it).
So which OS or phone are you going to go with? Google only guarantees 3 years of support. Fairphone 2’s last OS update was 14 months ago, there’s no way that’s up to date with security issues so their track record is looking pretty dicey. After all they’re dependent on Google as well. Even if they do eventually get another update there’s no way a 14 month gap between security patches is acceptable. Fairphone only actually guarantee 2 years of support anyway.
So what phone do you use that can beat the iPhone 6 with 6 years of regular patches?
You simply should not use a smartphone without security updates. It's as good as a brick in this case. (Or maybe never use Internet and Bluetooth on it).