I wanted to make a post in hope that it helps others out even if it's just a little.
For a long time I was having issues with the Spotify application when being used with Android Auto, the application would fail to resume play, wouldn't start playing or sometimes wouldn't even load and I would have to do things found on this forum such as having to force quit the application, clearing the app cache, signing out and in again, unplugging the phone and plugging it back in etc.
I've found a resolution it's simple and I'm stumped as to why I didn't try it ages ago, mind you I've done this about 4 weeks ago and not once have I had an issue since.
Simply make sure the Spotify application is exempt from any application optimisation your Android phone may be doing. Just search your phone and find out if it has optimisation for applications such as using power (battery), cpu, background usage etc etc and stop it.
I was using a Pixel 3, running Android v10 with the latest Spotify version, constant issues, a few friends of mine running Samsung Galaxy S10s and even a few Oppo users same issues. I've had them all do it and they all are having no issues with Spotify and Android Auto anymore.
Hope this helps someone.