Way easier to use a sata to usb cable and plug a ssd externally. It requires some technical skills to install internally. Did you know that unless your 2012 already had two hard drives installed, you'd need to get the mounting parts? Wouldn't be a great time if you decided to go internal, only to find out that you are missing parts during the install.
Although doesn't matter too much, usb 3.0 spec isn't 1000mb/s. The 2012 used gen 1, which in theory could go up to 625mb, but in practice it's about 400mb/s. This is still good enough for most people for most applicable uses.
You can absolutely go to 1000+ with thunderbolt on a 2012 mac mini, but the real question is, why would you bother.