I tested it Yesterday and this morning. The Navigation sound mix works fine for me with Maps and NavRadio.
In the Settings you can define a default Navigaton app. You also need to do that to make the sound mixing function properly. That already works this way since 2015.
Maps and most Navigation apps also try themselves to reduce the volume of other "sound producing apps" but do not always have the rights to do so. So always set the primay nav app in the Settings.
Sound mix does not work correctly with NavRadio and Google Assistant.
In all other players, the sound is largely reduces when Google Assistant is active. In NavRadio the NavRadio volume stays the same and I first have to pause NavRadio before being able to put a good command to Google assistant.
RadioProxy only works on 7862 and sc9853i units. Not on any other unit. This has been explained by the developer.
Besides: RadioProxy has absolutely nothing to do with the sound mixing.