There are some android TV devices that you can run LineageOS on which is what I do. But they are few and far between, and require specific firmware versions which makes it pretty impractical. I have used a Raspberry Pi with LibreELEC before as a frontend for Jellyfin and that worked decently well.

My dream is for Plasma Bigscreen to be supported well enough that I can just use a standard Linux machine for my TV, and there are tons of options in that ecosystem for casting, airplay, etc. But last time I tested it the issues were too numerous to ignore, specifically around application stability and UI navigation