Spotify does that. It suggests custom playlists (e.g. Discover Weekly) based on music you listen to. If you play one of your playlists, it will continue playing similar songs even after you run out.
Not the same - yes I want that too, but I want something that auto-curates my music by genre / artist / whatever. My key point here being, I want the option to curate, but I don't want to be forced to curate.
You're not forced to. You get a bunch of different tools in any platform with different degrees of curation. In spotify the most automated ones are Discover Weekly, radios for any track/artist, and daily mixes. You also get playlists made by other people (sometimes hard to find), but the browse section is never curated for you, it just has fixed playlists there. I have discovered orders of magnitude more music since streaming hit the scene.
And I'm fairly sure spotify's competitors also give you the same kind of options with different degrees of curation (if that is a word).
If what you are asking for is "I want a magic DJ that solves all of my listening habits" then I think no app will do that.
I'm finding that the Daily Mixes, found near the Discover Weekly playlist in the "Made for <user>" part of the app, do a good job of this, and will also include similar music not in your library for discovery
In fact, my personal experience is that Daily Mixes do a better job showing me new music than Discover Weekly does
