I couldn’t find the actual pinout for the 8 pin package, but the block diagrams make me think they’re power, ground, and 6 general purpose pins which can all be GPIO. Other functions, like ADC, SPI and I2C (all of which it has) will be secondary or tertiary functions on those same pins, selected in software.

So the actual answer you’re looking for is basically that all of the pins are everything, and the pinout is almost entirely software defined

BGA, like in the photo, isn’t the only option. There are options only slightly larger with hand-solderable packages (if you’re good at soldering)

How did you calculate that? The question didn’t even mention a specific speed, just “near the speed of light”.

The kinetic energy for a grain of sand near the speed of light is somewhere between “quite a lot” and “literally infinity” (which is, in a sense, the reason you can’t actually reach light speed without a way to supply infinite energy).