Ah, well, the biggest thing you have to worry about is that most vulnerabilities that would allow users to get ROOT access have been patched for old and new TVs, so you might want to be selective if you don’t want to do it the hard way. You need to know which TV OS you’re dealing with before you can look for the necesary tools.

The hard way is removing the SoC processor from the board, buying a custom mount for it, and using the debug pins to flash the OS. Most of the legwork has been done already for this method. www.synacktiv.com/en/publications/i-hack-u-boot

Something to keep in mind is that the processors in smart TVs are almost always pathetically slow. Also, the streaming services compatible with these TVs require hardware encryption so if your modified OS mimics the old one but isn’t verified then they will refuse to run. You would get better performance from using a computer connected to the HDMI port, or even a Raspberry Pi as your TV Box.