@scar-DJ - You cannot create (5) five duplicate posts!
Anytime I build the build is tested on a bench first, Tested & Benched, then installed in the Case. Certainly you can have a failed component, and in the end you might need to swap PSU or GPU to further narrow-down the bad component(s).
You can try Lazyman's Breadboard -
* Disconnect ALL Front Panel & USB Headers ; keep only the PWR & Ground <best> short w/wire.
* Disconnect ALL peripherals including the Keyboard/Mouse
* (1) Stick of RAM in the second DIMM slot from CPU
* Unscrew both the MOBO and PCIe screws
Pull the MOBO away from anything conductive and supported by a towel ; short PWR & Ground to start