Can you install arbitrary apps on androidtv or only what is available in the appstore for it without jumping through hoops that possibly involve some command line shenanigans? Similar to how you can install plugins in kodi in libreelec from the official repos perhaps but have to jump through hoops for arbitrary programs? Your real issue then is that libreelec’s ecosystem isn’t as vibrant perhaps and that it doesn’t run on locked down hardware the user doesn’t really control so the media companies allow high quality streaming to them?

Arbitrary applications isn’t a TVOS like the original poster asked for, it’s a general purpose OS.

Docker and Flatpack are both containerization technologies and work in similar ways under the hood. Docker is more geared towards running headless services that other systems access while flatpack is more geared towards desktop gui applications that are interacted with from the same system they run on.