pretty cool! how good are the songs downloaded with spotdl? Also ever heard of soulseek? You can selfhost that i believe.
A piece of advice : don’t use that shit!
Yes, you can make him an account and assign the space you want for that account. He won’t be able to access anything else that isn’t on his account.
Does it use activitypub?
mastodon is definitely simpler and more compatible. still, misskey is well-developed and more feature packed. it doesn’t get the love it deserves imo.
Proton on mobiles already exists Chech out Box64Droid