That seems to be only for streaming your own files and doesn't have an existing library. That's the benefit of Apple Music to me, I don't really have a full local library of all my music. I stream what they have, and then if I find something missing I'll source it from somewhere, upload it, and then forget about it.

I don't really want to give up the built-in music library cause there's a lot of benefit to that too.

It's a little bit more nuanced. It allows you to merge both cloud services like Qobuz/Tidal with your "local" music which could be on a NAS or even Dropbox. Not exactly what you're asking for, however, it's close.