You might want to consider setting up a Pi-Hole. Not sure how well those work with chromecasts though.

Chromecast uses Google DNS servers ignoring your DHCP DNS. You need to block Google DNS servers on your router so then the Chromecast will switch to your DHCP DNS.

Also, on my experience, some Youtube ads are served through the same domain that serves the video so you won't be completely ad free.

Pi hole cannot block ads inside of YouTube app, only on the web. I suppose that will be similar with Chromecast.

Youtube ads work in a way that domain blocking(pi-hole) cannot block them.

One thing is that it might be trying to get data from OG names to easier guess their passwords?