I have that setup. The entire front page of Chromecast is baked-in advertising with a small row of your apps and pihole with a good list still doesn't get rid of them, sadly.
Chromecast is built for and of ads. That being said, it definitely does "just work". Jellyfin + Chromecast is a great streaming experience. I don't have to deal with skipping and stuttering like on the android app.