Not sure if you mean hardcoded DNS IPs or hardcoded "phone home" IPs. Hardcoded DNS addresses in devices are annoying, the only way i've found to get around that is using destination nat rules (DNAT) which requires more than a consumer router typically. hardcoded phone home IPs would get blocked by your firewall. you're right that most firewalls are set up by default to implicitly allow outbound traffic. you set up a rule that explicitly denies all outbound traffic from the TV, then only allow port 443 (or whatever port your streaming service uses) on the specific IP/IPs that your service uses. Here's Netflix's published IP info for example.

edit also i'm fully aware it's fucking ridiculous that we as consumers have to go through this much rigamarole. you shouldnt have to be a literal network engineer to do something as simple as have an internet-connected tv that doesnt spy on you.