First, thank you for sharing. Your solution seems about as elegant as most around here!
Second, I have a question about how your rule below is working... What integration are you using on Hubitat for the Google Home Mini? Are you simply using the built-in Chromecast Integration (Beta)? Or some other solution...as this thread's Assistant-Relay does not support playing mp3 files. So, I am curious what integration you're using. Thanks!