@Mike_Maxwell Did you try the Bubble Upnp server? Like in the thread;
It can make the chromecast → audio ← be an DLNA and then be found using ‘Mediarenderer Connect’ app (RELEASE Generic Media Renderer (DLNA Speakers). Cheap Sonos Alternative (Update V2))
Credits for this goes to @adamrhoades
And then it’s reachable from smartthings as a mediaplayer 