I'm copper modding few GPUs I have and I need help with a tool that I can use to cut copper shims easily, I will drop a picture below showing the type of copper shim I got and im looking forward to cutting it into 15mx15m size.

copper pads is the worst thing you can do for the card longevity, too high risk of breaking mem chips or short-circuit power lines leasing to GPU death. Better just buy good 12.5W termal pads.