LG 65" OLED rarely powers on, turns itself off

I have a 2018 LG OLED65E8PUA.BUSWLJR that’s been in use for less than 3 years. It just began failing to power on. Sometimes I’ll hear a click 15-60 seconds after a power-on attempt with the remote (the power button does not work at all). The standby power light sometimes flashes and is occasionally followed by a picture on the display, but usually I can’t get a picture and when I do, it turns itself off after a while.

I found a free service manual download here:

https://encompass.com/shop/model_researc...

I’m glad I didn’t pay for it, because it is not written very well. The “No Power” flowchart on page 30 directed me to A9 “Power error _No power” on page 61. As shown in my blue annotations below, it isn't clear where I'm supposed to measure what voltages, and when (TV on or off).

From what I've read, this failure could be the main board, the power board, or the T-con board. I'd rather not have to buy all of those to find out which one it is. I have a DMM and oscilloscope with differential probes, but could use some guidance on making measurements to diagnose the failed assembly or component.