I think the best way forward would be a single board computer that can do an open source equivalent to chromecasting. Plug that in and leave your TV unconnected to the network.
You can’t do chromecast directly, because Google holds encryption keys for it. Unfortunately, this means casting apps need to be modified to support it.
There’s a few projects like this:
elrik@lemmy.world 3 months ago
I’m pretty happy with Chromecast currently for its simplicity. I meant to try and replace the TV firmware so it’s more or less a dumb TV that just displays its inputs without having ads and other gimmicks.