No you can’t use the spare M.2 for an additional SSD; you can replace current SSD with a larger one, though.
On paper USB-c offers good data transmision rates, so it could work. I think is also faster than using SD cards, which are limited to 90 MB/s. Just try and let us know.
I found a cheaper better option, 2tb sata hard drive connected with a usb c to sata cable…. if it works it’d save me a lot more money and i’ll have a lot more memory 🙂