As long as you are connected, I think network adb will stay active but if you leave your network f.e., you have to re-enable it in the developer options. But don’t take my word for it. Feels like google changed this behaviour every major release with android.

Oh, if that is the case, this will not work. I'll test it out later this week to see.

On their blog they say: This project is made possible by the awesome work of various open source projects, including Shairport Sync for Airplay, Raspotify for Spotify Connect and Snapcast for multi-room audio sync. So they “just” glue existing stuff together which leaves you with roughly the same limitations as if you would do it yourself. It might allow spotify to be used with snapcast for multiroom but as I’m a yt music user I didn’t digg any deeper.

Hmm, maybe I misunderstood it. Here is a blog post that shows how it can also use Bluetooth. To me, it sounds like that makes an app agnostic solution as long as you are fine with using Bluetooth. My understanding is that you then just connect to one of your speakers with Bluetooth when you want to cast, and you can then control which speakers the audio should play from. I will research this more. I should order a 3.5mm jack extension to the Pi Zero W, which is the only part I miss to be able to set up a proof-of-concept at home.