tokanada
That looks like a z seam. You can try to hide it in a corner of your models in your slicer settings.
It makes more sense now why you can’t get to 144hz.
Displayport 1.2 seems to be able to go up to 1440p@144hz when it’s 8bit color. This basically means any graphics card after 2010. You could probably pick up a cheap R7 450 or FirePro W4100 secondhand. Just make sure that the Nvidia drivers are removed and you’re running open source drivers before swapping. Afterwards, amdgpu is generally the best driver for the aforementioned GPUs.
If you need more than that then you’ll need a graphics card at least as new as the GTX 10 series or Radeon RX 400 series.
It looks like the Quadro P600’s displayports should support that resolution at that refresh rate. What driver stack are you running on your system?