Accessing copper is far from a primary goal of the BRI. Even if it was a goal--which sure China has their own interests in the BRI, who wouldn't--the selling point of the BRI that should be of interest to you is that vulnerable, undeveloped countries that have had no choice but being victims to Western imperialism for centuries are now getting an affordable way to develop without political stipulations or debt traps.

What China spends on their military, bottom line, is significantly less than what the US spends, not to mention all of the US's vassals combined. When we look at the numbers per capita, China spends a tiny fraction of what the US spends.