In general, I try to keep video streaming devices off the wifi as much as possible even though I have a pretty decent access point. Streaming video drastically decreases the quality of wifi service for all other devices on the wifi. Running an ethernet cable through floors is a bit of a hassle but can be worth it. A powerline adapter could be good for your use case, but they come with a lot of caveats.
It seems like the chromecast ultra is an outdated model at this point. I can’t tell if the 4k chrome cast comes with an ethernet adapter still.
They do seem to sell the adapter separately. I don’t know if it would be compatible with whatever unit you have. https://store.google.com/us/product/chromecast_ethernet_adapter_gen_2?hl=en-US
In general, I try to keep video streaming devices off the wifi as much as possible even though I have a pretty decent access point. Streaming video drastically decreases the quality of wifi service for all other devices on the wifi. Running an ethernet cable through floors is a bit of a hassle but can be worth it. A powerline adapter could be good for your use case, but they come with a lot of caveats.