Ok. Frustrated to have to revive this.
BT headphone connected to iMac. Music plays through BT headphones when the source from the iMac is any of the following native apps: Tidal, Qobuz, Apple Music, Spotify, anything on Safari or other web browser, including from the Tidal or Qobuz web interfaces.
When the source is Roon, music will only play through iMac speakers. I tried the ideas that I saw above, like adding the BT headphones as a source in Roon, but Roon doesn’t recognize them, and I don’t see a place to force that connection.
Even the Qobuz app sees the iMac speakers and the BT headphones as different playback endpoints.