I'm probably gonna have to buy a new PSU as well. Only got a 4 pin CPU connector and 8 pins on the board. According to the internet it should be fine to just plug half. It should be more than enough for a 95W TDP CPU.

Everything is in place now and the case is closed. Gonna turn it on now. If I don't post after this it completely blew up in my face.

Well, this is bad. RGB lights are on, CPU fan is spinning and HDDs are making noise. I get no display output. Removed the GPU and HDDs, still nothing at all. Mainboard System LEDs show nothing. There is only one orange LED at the other end of the board but that one isn't even listed in the manual. So it must be something with CPU or PSU? The search continues.

edit: Ok, the orange LED is apparently just the power LED, so that's fine.

The RGB lighting is the important stuff anyway.
You will be fine.

You can try your GPC power supply to see if that's the cause of the problem.

That's what I wanted to avoid. But maybe I'll have to.

By now I'm almost sure that my board or CPU demands an 8 pin connector. Apparently the CPU is important to even start the POST.

I'm gonna try to order a new PSU for tomorrow.

But you'll try the GPC power supply before ordering, right? I mean, why buy a new one when that may not be the problem...

You'll only need to fiddle with the CPU cable.

I want to replace the old PSU anyway. That's like 4 years old now. So I ordered a new one that's gonna be delivered tomorrow.

In the meantime I tried the PSU from my old PC which has an 8 pin connector. Sadly it's still the same problem. So I'm gonna disassemble everything again and see if I maybe fucked something up with the CPU.

I know it's a silly question, but did you make sure that your cooler was dry? Using water has its dangers...

It was fine.