I'm surprised no one has mentioned that KDE revived the Plasma Bigscreen project. No idea on the ETA but assuming all goes well I can see it becoming my daily driver very quickly.
Presumably locked bootloaders on smart TVs are a problem that would block usage of that project?
The problem with open source TV solutions like that is that they never support legal streaming platforms like Netflix, Disney+ and so on.
SteamOS/Bazzite also makes it pretty easy to integrate flatpaks into its gamepad-oriented UI. I hope that leads to the development of more apps that work with a remote control or gamepad, which would then also work on Plasma Bigscreen.