My TV is turning off a few seconds after start up

@blamman before you go all willy-nilly replacing random boards, I suggest you try some troubleshooting first. You need to check your power supply and measure the outputs to the main board and the backlight LED’s etc. Your TV may turn off because of issues on those other boards. sure it is possible that this is the power board but it may also be something totally unrelated. LG is a bit odd and a few even turn off if there is not response from the backlight or the T-con board. So, get multimeter and measure the voltages. You can try and disconnect the main board from the power board and see if it stays on. Measure stby voltage etc.

You also want to repost your pictures with some that have no motion artifact and proper lighting. It is impossible to see things clearly on the ones you have posted.