If you have a lot of space use 1mm which would dissipate the most heat but if you only have a limited amount of space then go for 0.5mm and I think you should read the reviews before buying the thermal pads

if its raised it would lower the heat transfer ability which would raise temperatures if its touching the CPU it would decrease temperature but each system is different experiment but keep in mind to use thermal paste

You can replace the motherboard but changing the CPU idk about that because laptops aren’t built to be upgraded so if you can change CPU I don’t think there would be a huge amount of CPU that would fit in the socket.