I just tried googling "mitmproxy chromecast" and found a bit of a rabbithole of hacks and tweaks that can be done to Chromecasts to alter their behavior in ways they were definitely not intended for :)
I expect there are probably umpteen different ways to block ads with a little digging, although I can't vouch for any as I don't have a Chromecast (or TV) myself.
FWIW, a while back I reached my eye-twitch limit with Raid: Shadow Legends (deeply impressioning irritating ads: ...why...?), and so I stared at YouTube's load process to try and figure out if I could viably block everything.
The technique I ended up using exploited the fact I was running within a Chrome extension and overloaded JSON.parse (lmao), and was specific to the HTML delivered for desktop, but has worked for months.
I reckon it's quite possible the data sent to Chromecasts is similar enough that you could viably block it by MITMing the device then rewriting the JSON (or possibly gRPC) responses being sent to it.
Using YouTube Vanced on a no-name Android TV stick might be an alternative. (Untested but should presumably/theoretically work.)
I don't feel like rewarding a company for grabbing a monopoly on short form streaming video then making their service borderline unwatchable through aggressive, increasingly unskippable ads.
Plus (getting back to the topic at hand), having adblock for all your devices is so ... pleasant. You forgot how jarring and upsetting (and LOUD) advertisements are. Having them puncture your DNS adblock while using Chromecast is like getting a wet slap in the face.
