Troubleshooting a Sony Bravia XR TV startup loop with manual machine

Have you tried disconnecting all HDMI cables and the Ethernet cord to see if an external device is causing a CEC conflict during the boot process? I spent hours dealing with a similar "black screen of death" on my old display, and I discovered that sometimes a simple firmware re-flash from a thumb drive can bypass the error if the OS has just become corrupted. I managed to find the full technician's breakdown on Manual Machine https://manualmachine.com and it was a total lifesaver because it listed every single blink code and the corresponding part that usually needs replacing. You should locate your specific Bravia model on their site and look for the "Service Manual" rather than just the "User Guide". If you can get it to stay on for even a minute, you might be able to run the self-diagnosis tool from the hidden service menu.