This 4K TV turned on with backligth but no video. Usual suspect in this case is the tcon board. Searching the part on Shopjimmy gets the part number 55.55T12.C02 and I was able to find the board for $20 USD on Ebay from Electroparts online. From the shopjimmy website, this panel is used Sony model XBR-55X900A as well.
With the new tcon installed, the tv was repaired. I did a factory reset and noticed that the default picture mode was set to Energy Saving. In the service menu, source-2-5-8-0 with the tv on, the backlight is set to 34/100 in this mode. In standard mode, it is 58/100. Backlights shouldn’t be an issue in this model if you leave it in these two picture modes.