Is copper thermite dangerous to use? What precautions do I need to take?
Any energetic chemical reaction is potentially dangerous.
Copper thermite, by design, produces an expanding cloud of molten copper droplets when it is ignited. If you have a body part, such as a hand, nearby when it ignites, you WILL be severely burned. There is simply no way to make an effective motor ignition aid that doesn't dump a lot of heat energy into its surroundings when lit -- doing so is exactly what makes them effective motor ignition aids!
That being said, with a few sensible precautions, it's possible to manage and minimize the risks, and mitigate the potential hazards of using copper thermite. Additionally, the instant ignition afforded by copper thermite can reduce or eliminate the risks associated with off-axis thrust due to partial ignition of motor clusters, or the risks of early ejection caused by burning off part of the motor delay while the rocket sits and chuffs on the launch pad before coming up to pressure and fully igniting.
On balance, using copper thermite properly can be less risky, overall, than not using it.