I'm not exactly sure what you're looking for with non-proprietary, open source artist music building platforms... maybe these two fit your bill. If not, they're open source, so you can definitely help make them fit your bill.

https://musicsideproject.com/
https://github.com/thebells1111/msp-studio

https://lnbeats.com/
https://github.com/thebells1111/lnbeats