Hi ejo,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
It seems you have already tried almost all the steps to fix the issue but the issue persists. I am afraid that your issue is more hardware-related rather than software-related. If its warranty is still applicable, contact the vendor to take a look at it.
Otherwise, make sure the issue is not somehow caused by the monitor. use another monitor or your TV if possible.
If the motherboard has a built-in graphics card chip, remove the external graphics card and let the motherboard use the built-in graphics card.
If you have more than one RAM, remove all of them and let the motherboard use only one RAM.
If the issue is unfixed, disconnect all unnecessary devices that are attached to your PC, reseat your old GPU, and press the power button for 30 seconds. Then, turn it on to see if the issue is fixed.
Please let me know if you need further assistance and keep me updated.