You don't need bandwidth to switch streaming services and import your old playlists though. For example a streaming service like Tidal tags all AI music and does not monetize AI generated music.

And if you weant the music straight on your phone, something like Opus 128 kbps is already considered high quality (we have come a long way since mp3). Let's say your cousin has 20 000 songs. That's about 60 GB in that form, which fits on most mid grade/flagship phones.