Honestly, my general experience has been reversed. I'm having varying levels of success in Chrome across multiple different PCs all on the same network on Ethernet but my experience with the chromecast (wired) has by far been the most flawless never straying from a reported "Excellent" connection.
Have you tried setting up the chromecast to use the same wireless network you said you had success with using Chrome? There could potentially be some issue with your network traffic specific to your hardwired connection related to some QoS you might not be aware of active or a misconfiguration of something like jumbo frames.
I'm having exactly the same issue.
Did you get it resolved?
I'm wondering if it's to do with the Stadia controller using a separate connection over WiFi rather than connecting directly to a pc.
I'm seeing the exact same thing.
I'm having great perfs on my laptop (chrome on macbook pro 16") but poor perfs (freezing every 30s) on my Chromecast ultra.
Same wifi (5Ghz), same distance from the router.
When doing a M-Lab test, I get 35 Mbps.