The problem is that music isn't generally FOSS - people would like to be paid for their work. So the next best thing is to set up your own music service where you can add your own music.
That's true. However, FOSS usually means that it's free (F) and open-source (OS). Music isn't usually open source - they don't usually share the sheet music or allow free editing/covering/forking of the music. So the worlds don't completely mesh. You can look at a FOSS solution for managing music, but you won't find a ton of FOSS music.
I don't have much experience with FOSS alternatives for Spotify because I rarely use the service, but maybe you could find what you're looking for in this list from alternativeto.net ?!
Yeah Spotify exposes their API only for premium customers.
Calling the project as a different skin is undermining them. The entire project was done to access Spotify without the messy bloat of electron. If you didn’t know electron, it’s just a chrome instance running in background masquerading as different app.
Alternatively if you have the music with you, you can Selfhost a music server and stream from that, making it your own personal Spotify, in a manner.