I haven't heard that complaint before about music fading in and out with Android Auto. I use AA extensively in my '19 and I've never had that issue. I also have various rental cars 1-2x a month for work and use AA in those each time and have not had the music fade in or out ever. That is with OnePlus and Google Pixel phones using Google Play music through AA primarily.
I have a 2019 Forester Premium use AA every minute I am in the car and never had an issue with anything fading out. That is with a Google Pixel phone. Instead of saying "Hey Google" you can also hold down the voice button on the steering wheel to prompt the phone to start listening for a command to send a text message or whatever.
My wife has a 2018 Forester Premium and it works the same way, except she is running AA on her actual phone with a phone mount and it works the same way I believe with her Galaxy S9.
I am a huge fan of AA and never had any issues with it in my 2018 Civic Si or my 2019 Forester.