Reading through what troubleshooting steps you've gone through you've definitely hit a lot of routes so lets check a few things.
you unfortunately don't have any Dr. Debug LED's on that motherboard from what i've seen.
I don't want to really believe the GPU is the culprit but do the fans spin to life at all when you try to first boot?
Have you made sure all cables to and from the PSU are seated properly?
have you tried checking the CPU's pins or swapped out the CPU for another?
if you cannot do the last statement (swapping), could you remove the cooler, clean the paste and feel if the CPU heats up at all with your fingers when you power it on? (you can do this just don't have the system on for longer than 15-20 seconds)
If none of the above, the worst case scenario off the bat is a motherboard fault whether it be a power delivery issue or something else.