I just faced the same kind problem The TV starts but shuts down after 4-5 seconds. Do not see any picture at all.
I started by investigating the T-Con board and found exactly the same "problem" that was explained here on the forum. 2 test points showed about 5 volts, although the legend text says something else. All voltages were fine. That's not the problem. Unfortunately, I found this thread after my own measurements.
I investigated the Main board and listened to it by ear and clearly heard one of the coil of the DC/DC converter making noise. This is exactly the same coil L2000, which was an issue in this thread. I measured the capacitors of the output end and their resistance was quite small, about 17 Ohm, and the reverse polarity was about 20 Ohm. The fault cannot be in the passive components. The measurement result was the same without the L2000 coil. The DC/DC converter is probably the Core voltage for the processor ~0.95V-1.2V (who knows?) It gives about 0.92V when it is on. The processor ASIC heats up strongly and the coil and DC/DC converter controller a little. My interpretation is that the processor is broken. This can be try to verify by investigating the IC8800 signals of the boot Flash, which are nicely available from the connections of the pull-up resistors. All other voltages seem to work correctly on the Main board.
As a curiosity, it should be mentioned that the voltages on the T-Con and Main board can be kept on by disconnecting the 2 High speed flat cables between the T-Con and the main board.
I would be very interested to hear if replacing the Mainboard helped with the problem? Spare parts seem to be still available, even though the TV is a 2018 model. However, an interesting investigation behind which cannot necessarily be calculated a monetary value.
Modified - some typos found.