It’s DRM free, you can download it with heroic, copy the files and then delete your epic account

AFAIK it’s a desktop environment so using it via distrobox wouldn’t work, it has to be installed on a system level

Stremio Youtube

You can use pretty much anything you can with desktop Linux, however some apps may not work with a controller

Jellyfin is quite capable for music, however Navidrome has a much better client ecosystem. Personally I use the Finamp beta on mobile as it does everything I want and is quite stable, but if you want Android auto/apple carplay you will have to use a client that isn’t as reliable or proprietary (paid.