IMO (!!!) Kodi (which is great of course) is allready enough non-gaming-content for retropie.
I am not interested in any streaming of music at all on the Pi.
As said: IMHO

RetroPie - on the Pi models - doesn't use the desktop at all and it runs as a dedicated application (EmulationStation) from the console. If you want to use your Pi4 as a desktop enabled computer, perhaps it would be better to use Raspberry Pi OS and install the same emulators that RetroPie uses and run them standalone.
You can start RetroPie from the desktop environment, but it's not a supported configuration and not something we recommend.

Hi, I´ve just got it today. Using the pixel desktop y following the steps of
https://pimylifeup.com/raspberry-pi-spotify/
https://github.com/dtcooper/raspotify
Just one thing, while playing, the game music and spotify sounds at the same time, I didn´t figure that up yet.

Greetings from Tucumán-Argentina