Music assistant is indeed cool and can do multi-room, but you have to go to it specifically first.

Snapcast allows me to use the “chromecast” button inside the spotify app?

Won’t work with multiroom unfortunately.

I have a home server, I just need the little parts that plug into the speaker.

I actually looked into Wiim and it’s really great, but you need to open the music in their app in order to do mutiroom. You can’t just “cast” from Spotify or whatever.