I'm not sure anyone prefers accessing their online services using the WiiM app vs the service's native app.
I wouldn't refer to the implementations as integrated, more like connected with basic control.
Music Assistant, which I'm currently testing to replace LMS, does have proper library integration for YTM along with Apple (in beta) and several other popular services and can stream to WiiM.
Music Assistant is a music library manager for local and streaming providers
music-assistant.io