Get a used mini-pc, install Linux on it, and don't allow the TV to connect to any networks. This is a 50-75 dollar solution. Good if you are on a budget and are not interested in any wiz-bang features like HDR.
There are a few TV-dedicated Linux systems out there, like libreElEC.
Or get a more powerful system with a AMD GPU and install Bazzite on it. That way you get something like "SteamOS for your TV". Pairs nicely with controllers like 8BitDo.
It would be nice to have TVs as open as PCs, but the manufacturers and media companies are ran by dirtbags and would rather have victims then customers.