I would honestly love to try switching to an alternative than continue using Fire OS on my Insignia TV, but I don't know of any good alternatives. Maybe I just haven't dug deep enough, but most "3rd party" (not Amazon or Google) Android implementations seem to only support phones, not TVs.
Fortunately, Fire TV can still install apps from outside their app store (like SmartTubeNext), but that can only do so much.