Help RIP Motherboard?

My pc decided to die yesterday.

The day before I fitted a bunch of Thermaltake Riing case fans (4 banks of 3 fans with one controller per bank) connected through a Thermaltake Commander FP fan hub so that they were drawing power directly from the power supply rather than from the motherboard header. After assembly and a tedious amount of wire management to make it all neat and tidy the pc worked fine. No signs of any issues, new fans worked great and looked even better.

Yesterday, however, it wouldn't start.
It powered on and off every ~20 seconds, endlessly. It got to the first, usual black boot screen and shut down, then restarted again on its own, and did that over and over until I switched off the PSU. On one or two occasions after it restarted itself, it'd shut down and restart even before the boot screen came up. It'd just flash the power light and start case fans for like 2 seconds, then shut down and try to reboot.

One by one I tried to disconnect all the fans and their controllers, fan hub, usb headers, audio headers, front I/O, hard drives and ssd, pulled out ram sticks one at a time, removed the graphics card, swapped power supplies, nothing helped. I unplugged and checked every single connection inside the pc. Found no bent pins, loose connectors or the like. As it continued its cycle of reboots, the '2 second blink' where it wouldn't even show the boot screen before restarting became more frequent.

I tried removing the CMOS battery for a while. After I refitted it, the usual boot screen changed to a red Gigabyte screen, which I assume is due to wiped config, but that was to be expected. Nothing changed though, it was still stuck in the reboot cycle.

When I ran out of ideas, I checked whether the motherboard was properly earthed to the chassis and that it wasn't touching anything that'd cause a short circuit. No findings. Checked for any wiring that could have been broken/kinked/cut insulation and the like, none found. The only thing I didn't look into was the cpu socket itself, as I didn't want to remove the heatsink especially as I wouldn't get thermal compound over the weekend.

Now it got to a stage where it never shows the boot screen anymore, just a flash of power light, starts winding up the fans, then shuts down.

I might be answering my own question here, but has the MB or CPU had it? Or am I just missing something obvious? I think I covered just about all the points

Hope someone throws a helpful suggestion. I'm getting into the phase of wildly staring at it and self inflicted hair loss!