You’ll note that I mentioned a proxy. That’s why: you need to force all traffic through a proxy you control or you’re just hoping that a client doesn’t use hard-coded IPs or an outside API of some source. If your network allows the client to send traffic to port 443 anywhere, any blocking is on the honor system.
My question here is how are you going to install your root certificate on a $300 smart TV? Or, if that is not required because the TV does not verify DoH certificates, how bad is that for security (which we already know is awful on these devices)?
This comes back to the core decision: do you care about controlling the network enough to block access? If the device can't be managed / is no longer supported the safest choice is not to allow it online at all. Different people will have different risk tolerances – it might make sense to put, say, a remote-control power switch on an IoT no-man's land network but if it has access to personal information or cameras/microphones it's not unreasonable to say it should just be blocked unless you're actively using those features.
