I've owned my 65" TCL Roku TV for 5 years. A few months ago I noticed the right side of the screen was a little darker than the left but still very watchable. Fast forward to today and the entire right side of the screen is almost too dark to see. I've done all the things people suggest but nothing has fixed it. Here's the rub... when the TV first comes on the TCL Roku splash screen is fine... full brightness all around. It isn't until the Home screen displays that the right side goes dim. Also... on occasion the problem changes from right side dim, to middle dim, and even at times left side dim... and then on rare occasions the problem goes away entirely and the full screen has normal brightness.
Don't let the occasional variations mislead you though... 98% of the time the right of the screen is too dark to see.