Google Assistant commands are not very useful if it never plays your music. "Play <song> on spotify" never plays something from my library. This is also annoying when there are several recordings of the same piece by different performers.
To improve google assistant I recommend adding this command:
"Play <song> from my library" this will then search only your library/downloaded music and play the closest match.
"Play <song> preformed by <artist>" For example play "Sibelius's second symphony" preformed by "Berlin Phil" this can be implemented for general search, but could also be combined with my other suggestion.
It looks like your idea was posted a while ago but, unfortunately, it hasn't gathered the minimum amount of votes to stay active in the Ideas Exchange (50 votes in the first 3 months). I'll mark your idea as "Closed - Not Enough Votes". You can learn more on how the Ideas Exchange works here.