As for Spotify: if you’re using a Pi as bridge, you can achieve this by installing RaSpotify in the Pi. Your Pi will be visible as a regular Spotify Connect endpoint in the Spotify apps.
The value assessment of Roon is yours to make: while many here consider Roon to be a great and – relative to hardware investments – low-cost improvement of their Hifi experience, Roon obviously needs a sizeable local library and/or Tidal usage to fully bloom.
In the words of a six-stringed hero: It’s in the way that you use it…