Well, as I was said, Spotify is hard to handle but this feature request is really not hard, because by my own observation it is doable.
- There’s an inconsistency between lullaby selection lists in settings and when you choose a lullaby from tapping lullabies during sleep tracking. Settings do not offer the same list. It should be unified.
- I can start streaming Spotify from phone to TV, Sleep as Android knows that, because it shows a progress but… it cannot handle volume for the specified period of time. Adjusting volume manually from phone shows that the volume is being adjusted in the cast. So, I believe it is doable from the app. No Spotify hacks, just adjusting the volume of the cast.
- I can start a standalone Spotify music on TV as a lullaby. The app on my phone understands it as well. And probably volume could be adjusted as well.
At least I am sure you could do the point 2. To adjust the casting volume.
As for the turning off my TV after the Spotify lullaby ends, perhaps, it’s out off topic and doable through Automate and Home devices. I won’t dwell into the details.