My car isn’t fancy, so I was previously using Android Auto for phone screens, but Google is trying to kill that off in favor of Google Assistant driving mode. It seems to work fairly well for what I need it to do, but it doesn’t appear the Plexamp integration works while the main Plex Android client appear to be fine.
You can access Plexamp from the Google Assistant driving mode interface, but you have to open the app separately to be able to get anything playing. See the screenshots below.
But if you select the regular Plex Android client from the list, you do get some options for playback!


