Have not tried it but came across this some time ago. https://github.com/guysoft/FullPageOS

I have not yet found a solution I would be happy with because you always have to sacrifice on something. Your scrpcpy idea is neat and something I haven’t though off but:

1. others (visitors etc.) wouldn’t be able to “just use it”
2. it would drain the battery
3. on none-rooted devices the adb network connection is not permanent I think? either way, getting the connection is not as simple as “pressing a button to cast”

As it stands, I have given up on integrating mobile devices into my multiroom audio setup for many reasons. Right now I have my HTPC as the snapcast server and RPIs (soon to be old surface 4) as clients dotted around with a special script on the server-side to do some magic in regards to Kodi.

By chance I came across fcast which sounds interesting but relies heavily on adoption which you will not see in apps like youtube, youtube music, spotify etc. so (to me) it has no relevance.

Android App is just for controlling