If you want to self host such a platform, how should it inform you about new releases? If you haven’t downloaded everything on your server manually beforehand, where should it take the media file from?
Just use the streaming service of your choice. Self hosting music is over.
You seem to look for „digital signage“. So rather Google for that, imho. We use https://www.xibosignage.com to show some stuff, you are relatively free to put what you want where you want and can do that centrally for one or a lot of displays.
You won’t find one, because these solutions need to interact with the networking hardware. I would indeed suggest UniFi as well, because they have great, affordable, controller managed access points and switches. Normally, that only is a feature of multiple times as expensive enterprise gear.
That is not at all how this works!
No commecial song is available “for everyone”, every song has royalties that need to be paid for it to be legal streaming.
None of these services support downloads, except in their own app, but then encrypted, so that no other app can use it.
There are some “Playlist curators” see https://routenote.com/blog/best-third-party-spotify-playlist-curators/ for some. But only if you have an account with the service.