Do you believe it make sense to create an entry in the Wiki to help to identify the issues the user encounter with Google Assistant? At least, it would help to write useful bug report.
Problem solved:
Personally, vocal commands works perfectly with Android Auto. However, using the usual Google Assistant, either it asks to choose a music provider (obviously Symfonium does not appear in the list) or it tries to use Youtube Music.
Brought benefits:
I feel issues with Google Assistant are frequent. In addition, Android provides many kind of voice entries (Android Auto, Google Assistant, Google Home, Gemini, Voice keyboard…), it is not easy to have a clear understanding of the issue.
Here a few notes for people who has issues with Google Assistant (correct me if my assumptions are wrong):
Gemini does not work with Symfonium. You have to select Google Assistant (GA).
Users reported you should be able to open Symfonium with “Hey Google, open Symfonium”. However, I can’t get this behavior. I suspect this is because French language is selected.
Probably because of the previous item, “Play XXX with Symfonium” neither work.
If you want to be able to play music without specifying “with Symfonium”, you have to select “No default provider” in Music settings of GA. Then, users report GA use the last opened music application. On my side, it sometime works with Android Auto. However, I rarely get it with without Android Auto. GA tends to continue to use YouTube Music, anyway.
To force GA to use Symfonium, you can uninstall (or disable) all the other music provider. Especially, disable YouTube Music and YouTube (Settings > Apps > YouTube > Disable). This workaround is acceptable for me.
To summary:
Disable Gemini
For every active language in GA:
Check if “Open Symfonium” works
Check if “Play XXX with Symfonium works”
Set “No default provider”
Check if “Play XXX” properly opens the last active music player with Android Auto