Constant reboots / resets after POST

So, I bought a new computer - been using laptops only in work for last 7 years, so I splurged a bit.

Specs:
AMD Ryzen 7 5800X
ASUS ROG STRIX GeForce GTX 1660 Super 6GB OC <- only Graphics Card I could find, right now...
Corsair RM750X V2 / 750W / 80+ Gold
Crucial MX500 2TB SATA
G.SKill Trident Z Neo RGB 32GB (2x16GB) / 3600MHz / DDR4 / CL16
Noctua NH-D15 chromax.black
ASRock X570 PG Velocita

Chassi is Define S.

So. Apparently they add an extra power cable to something every time I build a PC, but after that little thing, everything was working fine. I am using Ubuntu for work, so I installed 20.10 on the HDD, everything worked fine. Everything. No crashes, no problems.

But I also wanted to play some games. So I used my old 500GB SSD to install Windows 10. A legitimate copy of Win10, bought a key and everything. Fine.

Installed some games. Loaded up my old BF4, haven't played it on PC for many years. Everything worked perfect. Latest drivers, everything is nice. All settings on Ultra, the fans didn't even accelerate, so great!

Then I took a break. I had realized I made one error - Win 10 had become the default boot option and to boot Ubuntu I had to press F11 for boot menu, which worked, but was annoying. It didn't cross my mind to change the order in the UEFI settings in BIOS, but that's (probably) neither here nor there.

So I installed reFind Bootloader, which simply manages your different boots and is nice and has worked perfectly for me before. And it did this time as well, simple install from inside Ubuntu and I could now go back to playing some more BF4. So I did.

I must have played between 12-18 hours in shifts of perhaps 2 hours at a time during this first running of the PC, everything was very nice.

Then the PC all of a sudden crashed. Just black screen, hard reset, inte middle of a game.

When it restarted it got stuck in a boot-repair-reboot loop. Over and over again.

I've been struggling with this for hours now. I need some angles, insights and tips on this.

Did I burn the CPU to a crips? They're supposed to have overheating protection and the computer starts and I have managed to enter a Windows repair console once - then it did a reset again when I did bootrec /scanos - it did manage to do some other things before that, but they didn't give any indication of what might be wrong.

Things I've done: reassembled everything, checked every screw, every riser, every power cable, tried reseating the CPU two times. Tested the computer with an oooold graphics card - same behavior.

Tried booting the windows install again, came to the partition selection thing, using a mechanical 2.5" 500 GB HDD I had lying around. It crashed when I had selected the partition and pressed Next to move on to the next step.

Tried booting a live USB of Ubuntu. No dice, crashes.

Currently I have removed the CMOS battery (I have used the clear CMOS button 100 times during the last 8 hours, I have no idea why this would make a difference, but some people claim it does, I certainly hope so).

So, right now, I am so bummed. Nothing works, not one single boot option works ( will set up a command-line only ubuntu as some kind of effort, but I don't get what could have happened to the GFX card).

Any advice on where to being would be appreciated. I have sh*t Internet, so downloading all the games and stuff took a lot of time, but who cares? I haven't lost any data or anything, since I was just getting started... but I want to use this beautiful machine, not poke around in it like a madman.

//TomTom