FWIW: There are some online tutorials/videos about replacing/fixing the backlight on TCL tvs.
I agree with you that they're cheap tvs. Onn too. I think Roku is hurting itself by trying to support such a wide variety of tvs (hardware). I think that leads to updates not being tested. Then waves of disgruntled customers complaining about how an update broke their tv. The whole thing (not just TCL) seems like an incredibly bad idea.