Aha, thank you for clarifying. It’s easy to overlook the difference between “Chromecast” and “Chromecast with Google TV”. Unfortunately, it looks like if you want 4k you are stuck with the Google TV interface. :/[Edit: I was wrong, see below]
What type of Chromecast do you use? I recently bought a Chromecast Ultra for a new TV after being happy with a secondhand one for years (3rd gen, I think). The difference in UI was such a disappointing step down. I don’t want a home screen with apps and ads, I just want something I can stream to from my phone! And I can’t say for certain, but it also feels like I get more ads on YouTube compared to using the older Chromecast.