Ah, right.
As we know, Android Auto then Google Maps can be a pain at startup taking a couple of minutes at times. But, once connected and running you should be able to use the Home button and use the headunit as you normally do.
This allows the connected Google Maps to be in standby and switching to it should be possible in seconds whilst on the move.
I haven't tested this on the move but I have had instances, in the garage, when I've gone out to try something to answer a post. Google Maps has fired up and sat in standby whilst I fumble around other areas.