Wouldn't it be more effective if a huge corperation like Spotify themself contacted artists Saying:
Hey there (artist on Spotify), we'd like to inform you that you have music such as: (Songs) that your fans would love to listen to but isn't very accessible. Why not put it up on Spotify!
Thank you, sincerely Spotify
It would also help build loyalty. If I were contacted by Spotify saying they want more of my music on their platform and not just that if the letter included song names, I would flip out and absoluetly love spotify be loyal and upload everything i ever put out on there.
Not trying to tell you how to run your business but just a way to get all music of an artist.