This is a seriously left-field suggestion, because, it's neither a streaming service nor a recommendation algorithm, but over the years I've never found anythin

This is a seriously left-field suggestion, because, it's neither a streaming service nor a recommendation algorithm, but over the years I've never found anything better than last.fm for classification of music. For as long as I can remember, last.fm has had the ability to show you similar artists when given any one particular artist. And it's remarkably good, in my opinion. With it, I've discovered so much great music that I'd have never stumbled upon organically. It's also totally free to browse and without signing up. For example, browse artists similar to Jean Knight: https://www.last.fm/music/Jean+Knight https://www.last.fm/music/Jean+Knight (scroll down to "Similar Artists", or just tack on /+similar to the URL)