@valentinna97464 this definitely sounds like a backlight issue. It is very difficult to confirm since we would need to know how many amps your backlight pull from the power supply etc. Based on the flashlight test and based on what you are reporting, I give this one a 99% chance of being a failed backlight strip.
And is it worth fixing this model? Backlight strips for this TV might run you anywhere up to $40USD. Your labor cost will be 0 since you are doing it by yourself :-) Saving the TV from the landfill is priceless. So, can you purchase a replacement for $40USD? Most likely not. That means its definitely worthwhile to fix it