How to get Assistant to look for music only on my phone

I have about 60 gigs of music on my Samsung Galaxy S8. I normally uses Pi Music Player, and am very happy with it. But that's not so convenient while driving, so I'd like to control my music through Google Assistant. However, when I ask Google assistant to say, "Play John Prine from my library", it's hit or miss whether it starts playing my John Prine collection or whether it offers up some random John Prine video from YouTube. I can't seem to find any way to tell it reliably that I only want music from my phone. I was about to buy a new car and enjoy the pleasures of Bluetooth voice commands so I don't have to fiddle with my phone while driving (my existing ten-year-old car does not offer me this option). But it now appears that there's no way to do that; unless I pick up my phone and use the touch screen, the system is going to play whatever it feels like playing rather than the music that's right there on the phone. I've put the car purchase on hold until I figure out whether there's a solution to this. (I've also just spent an entire very frustrating day trying and failing to find settings I can adjust...) Is there in fact a solution?