AAstarter
This app waits for a trigger from my fork of AAGateWay and starts Android Auto instructing it to connect back to AAGateway.
You can build it or get it from the Releases section and you have to install it on the master phone.
Play Protects kills the app
I noticed that Play Protects stops the app during its periodic scan since it considers it a potentially unwanted application.
I'm not going to sign-up to the google developer program just for one single app, I'm not going to disable play protect either, so I solved the problem with an Automate macro that starts the service provided by the application when it connects to the slave's hotspot.
Any suggestion is welcome on how to disable play protect just for this app.