OsmAnd works fine in Android Auto with WiFi and mobile data turned off. Sygic does too. I believe TomTom also sells navigation apps that will work fine under these conditions.

I use Android Auto mostly because I don't trust manufacturers of car components to maintain their software and to put more than bargain bin SoCs in their infotainment consoles. There's no need for your Android phone to have a connection to the outside world if all you're using it for is locally installed apps.