Great question, first off did the voice turn by turn directions ever work for you using Android Auto?
You can try taking the following steps to resolve your issue.
Launch your phone’s Settings menu and navigate to Apps & notifications > App info > Maps > Storage & cache. Now tap the Clear cache option to clear the Google Maps cache. Relax, that will not delete the Google Maps data. Relaunch the app and see if voice navigation works.
If that does not work for you, then take a look at this article here. It shows you how to enable the turn by directions for Bluetooth specifically.