It looks like it might finally be happening - the ability to choose a default music service for Siri:
According to the Reddit user whose image that is once he selected his choice once, Siri used that as default from then on. Also works with CarPlay according to the Reddit comments.
I hope it’s what we think it is. I’m ready to move back to Spotify for its better (imo) music discovery and the family plan I chip in for but can’t use.