Nice review of the system, some useful feedback for Volvo.
You can use USB stick, need to download audiowagon, details on link below.
https://moleman1024.github.io/audiowagon/
For bluetooth streaming, it will normally pick up from what was last played on your phone (does with mine ,Apple phone) you just need to press the play button the centre screen if it does not play automatically, (i just have playlists and it plays from that)
Do agree that the Volvo app is not great and a big backward step, hopefully will improve in the future.