Cool, thanks for cross-posting here. I really like how seemlessly iris integrated into the HA web interface. I had always aimed to do something just like for Pi Musicbox (RIP) but I should have just used HA!
A few suggestions:
As previously mention I installed mopidy on a PI3, because the spotify library isn’t available for the PI4
libspotify has always worked on Pi 4, providing you use the normal 32-bit version of Raspberry Pi OS. To install Mopidy-Spotify the easy way (i.e. not the way you’ve documented) install from apt.mopidy.com and then sudo apt install mopidy-spotify.
To run an application that access spotify you have to go to this web site login and create an app.
You don’t need to do that. The client_id and client_secret you require are obtained from Mopidy-Spotify
The versions of Mopidy, snapserver and snapclient from Debian archives are quite told. You are much better off using the latest versions. For Mopidy that’s installing from apt.mopidy.com and for snapcast that’s detailed here.
I’m a little surprised you don’t run into problems as all your copies of Mopidy are using the same caching directories. Despite your efforts to create one each in the service unit, you didn’t tell them to actually use it e.g in /etc/mopidy/mopidy1.conf you’d want:
[core]
cache_dir = /var/cache/mopidy1