I'm curious to know if you've looked at the PC 2.0 spec for music playlists: https://github.com/Podcastindex-org/podcast-namespace/blob/763bf928e5dc0b7de6fcf3e30f2630ef05f233ac/docs/1.0.md#examples-17 . Might be easier to leverage the work they've already done to build playlists.