Help determining cause of issues? 8 Red Flashes?

The exact model is XR-55A80CJ.

This is long so bear with me. TL;DR at the bottom.

Original issue: No screen. Random behavior when trying to turn on. Diagnosed faulty/unreliable main board.

Description: Power board supplies proper, steady/stable power across all relevant rails to both Main and T-Con Boards. When plugged in, standby light comes on (system is off). There is a relatively loud, high-pitched coil whine emanating from the power board. System does not respond to remote in any way. Power button on the main board does nothing on a press. On a hold for 10+ seconds, SOMETIMES an indicator LED (not the standby LED, a different, pinkish LED a couple inches to the left when facing the TV screen) will flash, other times NOTHING will happen, and other times the standby LED will turn off.

If the standby LED turns off, three things could happen. First, nothing could happen for about 15 seconds, then the standby LED turns back on and we're back to square 1. Secondly, the standby indicator could turn off and NOTHING happens and nothing changes this, both the remote and the main board power button do nothing. Finally, I believe the TV is "on", the coil whine from the power board subtly changes and it responds to the remote control in this state. But all input from the remote control only makes the indicator LED flash once per press, or repeatedly if held. Every flash of this indicator is paired with extra coil whine. Every button on the remote delivers the same result, with the exception of the power button, which does nothing at all. When in this state, pressing the power button on the main board does nothing, but holding it for 10+ seconds turns the TV "off" and the standby light comes on again.

Under all circumstances, the screen itself remains completely off.

In all cases, it is reset by unplugging the AC, draining the capacitors, waiting a minute, plugging it in, and waiting a minute. After every reset, any of these outcomes could happen, and it's seemingly random which.

This behavior does not change in any way if the T-Con Board is unplugged from the Main Board and/or Power Board. Nor the wifi adapter or speakers. When the Main Board is disconnected from the Power Board, the coil whine is almost unnoticeable. Of course, all unplugging/replugging different components was done with the unit unplugged and capacitors drained (as possible).

After seeing all this, I concluded the main board was faulty. Namely, the power board was delivering steady, consistent, reliable power everywhere it was supposed to as far as I could tell, and responding correctly to the power on signal.

So I ordered a new main board. Main board installed and instantly I can see by both the coil whine and the Standby LED that the behavior is different (and what it should be when operating properly) so I think the problem is solved. But the screen still never turns on. And after some time the Standby LED starts flashing 8 red flashes.

New issue: No screen. Unsure fault.

Description: Power board supplies proper, steady/stable power across all relevant rails to both Main and T-Con boards. When plugged in, standy light does not come on (system is off). Coil whine is present but significantly quieter. Pressing either the power button on the main board or the remote (intermittently, i.e. it may work on the 1st try or on the 10th try) can turn the TV on, where the Standby LED turns on, then turns off again. I believe this is "on". In this state, pressing any button on the remote makes the Standby LED flash. Pushing the power button turns it back to the "off" state. After an indeterminate amount of time (i.e. sometimes after 1 minute, sometimes after 15 minutes), the Standby LED will begin flashing red. Sometimes it is lit white while it flashes red, and sometimes it is not. It is a pattern of 8 flashes followed by a 3 second pause, as indicated on Sony's support website. Searching for this code only returns vague/generic "hardware fault" and nothing conclusive.

The TV may start flashing red either in "on" or "off" state, it does not matter which. After it starts flashing, pressing the power button on either the remote or main board will reset it and try to turn the TV "on", again with intermittent amount of attempts of pressing power before any results happen. The cycle repeats indefinitely until it is unplugged.

An additional note is the very first power on with the new main board I got sounds from the speakers. Soft "bomp" sounds that accurately corresponded with pressing the power button. This never happened again after the first time it was powered on with the new main board.

I figure it is possible I got a faulty replacement main board with a completely unrelated fault, or there is something ELSE wrong with this TV in addition to the main board. After everything I'm seeing and have observed, the behavior going back much closer to "properly working" and even delivering a legitimate (if unclear) error code confirms to me that the old main board was faulty indeed, but after that I'm unsure.

This is an OLED TV, not an LED, so there's no independent backlights to be burned out and need replaced. The TV screen has been carefully inspected and has no damage/cracks/etc.

A little at a loss and hoping someone else has more experience with OLEDs in general and/or has better documentation on what this error code means that Sony will tell me.

TL;DR: Screen remains off no matter what. Old Main Board presented random symptoms. New main board appears to work in all ways except no screen, before giving the vague "8 red flashes" error code.

EDIT: A second new main board and putting all the screws in and the problem was solved.