Interesting. My method for finding new or similar music to what I have in my library is to use TasteDive. Crowd-Sourced, so you get a 'real world' recommendation. It can be a little bit of work, but I find it quite effective. TasteDive also works for movies and a lot of other things. It does have an API tho I've never explored that side. I'm not sure what software would interface with their API.

I also hopped back into ListenBrainz and Last.fm within the last year. Exactly like you said--they're not trying to sell something, so it was interesting to go back and see just how much that influences the recommendations.

The suggestions are better, I'm building up a local library again, and not paying for Spotify. What's not to love?