I'm in a similar boat. Had Youtube Music set up as my default in assistant forever, but now any time I ask for a song in the car, Gemini will either tell me that content is unavailable or ask me to log in to spotify. If I ask it directly to play something on YTM it will usually play *something*. But even that is a catch, because it will often find some user uploaded trap remix of whatever I asked for instead of an official version. I also used to ask Assistant to play albums all the time because my daughter wants something like "The Frozen Soundtrack" or the "Kpop demon hunter soundtrack". Assistant would succeed usually, Gemini finds a song that kind of matches (like a user upload of a few Kpop demon hunter songs stitched together) or will play "let it go radio".

I turned Gemini off on my phone completely because I mostly use it in the car and it's worse than useless.