I'm not sure about size limitations, but apple music allows syncing personal collections through their service. Having access to my large library, much of which is not available through any streaming service, is why I use it.

That said, apple music has a large number of bugs with this service that are infuriating to deal with. For instance, I've noticed it dedups some of the library to the incorrect (censored) version of songs.

> I don't believe there's any service that offers that

Like I said, Apple Music literally does offer that. I use it every day. It’s just got problems.

Google Music used to offer it.