But tons of stuff would have to get sync’s every time you connect your phone. Better to have them cached, encrypted at rest, decrypted by key stored in the phone, and just do a diff-sync.
This should be very easily possible with CarPlay and Android Auto. I have no idea if it does or not. But as Apple and Android both control both their respective app and the OS of the attached phone, there’s no reason it can’t (and even pre-compile diff packages for known cars, or expire and purge both sides after X days without a connection)
That may not be true for regular old Bluetooth though…which likely has more to gain in performance from caching the resources due to BTs limited throughput, but also has to conform to standards.