I don't think that's it because when I tried casting YT from an account that doesn't have Premium, I get way more ads, i.e. 10-20x as many, and much longer ads,

I don't think that's it because when I tried casting YT from an account that doesn't have Premium, I get way more ads, i.e. 10-20x as many, and much longer ads, just like in the app, so maybe 100x the total ad time. When I cast with YT Premium they're literally 2-3 second ads, and they occur very rarely, so it looks to me that these ads are specifically "calibrated" for YT Premium subscribers, maybe on the order of 1% (or less) of the total ad time of a basic account. And they only occur if I've watched a lot of YT, maybe over 3-4 hours in a day. I don't understand why Google thought it would still be worth pissing off Premium accounts for this little ad time.