I just cranked up Roon, having been left on and idle overnight. Turned on TV, which is correctly set to the Chromecast input. So system-wise this should work and did work yesterday.
Now, no Roon Chromecast, just the Chromecast screensaver. Disable/reenabled, no joy. Haven’t gone further, but that’s beside the point.
Thought Roon would want to know.