> (but ads will play when watching YouTube unless you have premium obviously)

Why obviously? Can't you use an adblocker with Chromecast? Isn't it just a dumb IP video streaming device?

I've never used or seen a Chromecast, but if you can recommend a free-software/cheap alternative i'd be curious to try it.

Nope, it's a tiny embedded device you have little control over. It's slightly more intelligent than just IP video streaming ( e.g. there are app integrations) - if you cast a random video, it will play that video; if you cast YouTube or Netflix it does stuff directly with them and there are ads on free YouTube. DNS adblockers don't work by default either, since Chromecasts have Google's DNS servers hardcoded.

Holy crap it's worse than i thought! Does it do DoH or can you just configure reroute all DNS trafic on your router to your local resolver/pi-hole?

I think the reality is there is no free-software alternative, because the entire user experience is based on ecosystem buy-in where every media source (Netflix/Amazon/YouTube/Hulu/HBO, even NPR/PBS and Chrome itself) all actively have support for the Cast flows within their products (some only in their native apps, but many in their web experiences as well).

I did a quick search on Github and i see there's a variety of clients implementing the Chromecast protocols, but no server?! Is there really millions of such devices using a proprietary protocol noone has written a free server for?