How to use Google Assistant in car while paired to Bluetooth, but without changing audio input to Bluetooth?
If you have the newer android 7.0 OS version, yes there is a way:

Go to - Settings -> Sound and Vibration -> Separate App Sound
Once there, tap "app" and select "add application", then look for the google assistant app (labeled "Google" with the "G" trademark icon that has a small inner grey circle around it within the app icon bubble).

Set this app up with the audio device "phone", and it should work solely through the phone speaker/mic from then on, even when your car Bluetooth is connected.