Chosen Solution
@tronz yes you do want to go ahead and measure the voltages. Also post some good pictures of your board with your QUESTION. That way we can see what you see. Going by your description I would focus on the power board at this time. Just to verify, you did say you already replaced the main board. Reflow may not what your TV needs. You should go ahead and do some more troubleshooting. First step I suggest is that you check the standby voltage on your board. You should have something like 3.5-5 V on it. Check PS-ON (pin 1 on P201) when you push the power button. Does the voltage go up and stays there or does it drop after a few second. That would mean an error has been sensed. If your power board is a LG EAY62169901 turn it over. Your voltages etc. are listed on P201. 3.5V_ST (3.55V) output P201 pins 9~12 12V output P201 pins 17, 19 and 21 24V output P201 pins 2, 3 and 4. Not sure about the connectors that you are describing as tough to remove. LG connectors commonly have tabs on either side that need to be depressed to remove the cable.
Adding images to an existing question Update (07/09/2021) Here is the
troubleshooting chart for your TV.
