Turns out you can't stream anything to chromecast audio devices without a YT Music subscription (desktop/Chrome doesn't have this limitation).
So I normally shrug at the Google abandonment threads as software is mutable and everything goes away eventually. But this is an artificial limitation that forces us to lease access to the hardware we've already paid for.
Not thrilled in a large way. Next phone will probably be an iPhone, next speakers will probably be Sonos, and the Pixelbook is likely to be my last Google hardware chromebook.
(Solution: move music to a QNAP NAS and stream it from there.)
What are you using for streaming your owned music these days?