You are correct, it is quite difficult to tell at this stage what could be exactly causing the issue.
Have you been able to try with a different power supply?
Have you tried with different processor, if available?
If you have made any changes in bios, for testing purpose, please can you try to set bios to default settings and see if there is any difference in the booting?
Did the problem started happening after you updated the bios?
If the system was working fine before the update, is it possible for you to downgrade the bios to the previous version you were having?
If you have any front panel (audio, usb), please disconnect it and test the system.
If possible, test the system with different chassis and also most importantly with different graphics card.
If the above suggestions did not help you in any way, it could be something wrong with the board, but it might be advisable to open a service ticket with Intel support and they may investigate if required.
Hope this will be helpful.