you're not casting from your phone or laptop are you? I've noticed that quality when casting are much more limited than if I load the youtube app by itself on my TV and navigate to a video and start it on its own. What I actually do is play the first 3 seconds so it populates into my history on my phone, then navigate the android TV app on the TV to the history and play from there.
Try the smart youtube tv app.
Stock YT app switches to 60fps automatically if a video has 60fps option. You can turn on "Stats for Nerds" to confirm it.
Smart YouTube TV allows you to force resolution,codec, and fps at the beginning of a video (again if video supports it), like 1080p AVC 60fps or 720p VP9 60fps. Its dev blocked all ad servers he knows of. If any ad slips through, file a bug report on GitHub and he'll try to figure out which server ad comes from if he can replicate your problem. Funny story is dev himself saw ad slip through in the past. I haven't seen any ad for a long time.