On AA head units:
1) Texts will pop up on screen. You can either touch the screen to read, or ask Google to read them for you. In my 2019 vehicle you only get multiple texts if you have not read (listened), replied to, or dismissed texts received since connecting to Android Auto.
2) To eliminate "double notifications", you can turn off the "ask for access" notification and never deal with it. Additionally, you can turn off message access in Bluetooth settings (on the phone) for the Factory infotainment side; if you plan on using AA exclusively for messaging.
On non-AA head units:
1) You are in fact using your phone as the selected source (Bluetooth audio) for audible alerts to be heard. Optionally, you can have all audio routed through the phone speaker itself if listening to other sources via the vehicle's infotainment system. Effectively, you can listen to OTA radio while still using AA on the phone for texts, maps, Google Assistant, etc.
Bottom line: while I understand and can empathize with your frustrations, the reality is that Cortana & Windows OS for phones are no longer; so one must adapt & overcome! :wink: