Impossible. Spotify Connect is a headless Spotify client. It is not a local network streaming technology.
Spotify wants to own the user experience and interface for their users, and Roon is all about the Roon interface. Those ideas are incompatible.
Sonos market share is the only thing that got Apple and Spotify to bend. Roon is doing just fine, but we do not have Sonos’ user counts.
What are you using Roon for if you don’t want to use the Roon interface? What is Roon to you without it’s interface?