The rule of thumb is to shoot the Milky Way when yiou have a dark sky. In a dark area the full moon on the opposite horizon is still fine. A new moon or lack of a moon suggests light painting - as in JRP's image.
As suggested, for a night sky with stars - and not trails - you want to use an exposure of 500 divided by the focal length. That's 20 seconds with a 24mm lens, 30 seconds with a 16mm lens, and 10 seconds with a 50mm lens. These shutter speeds avoid star motion.
ISO should be at ISO 1600 to 3200 - not higher unless really needed due to noise.
Set the aperture at whatever it takes to make the exposure work. It will be pretty close to wide open - maybe stopped down a very little bit.
I start with ISO 1600, f/3.2, and 20 seconds using a 24-70 lens at 24mm. I try a test exposure then go from there.
I had no problem using the center sensor to focus on the brightest star in the sky. Orion is bright enough in a dark sky with the center sensor. If you are light painting, focus on the object you are painting. The stars are bright points and will be fine unless they are really out of focus.
Shoot RAW and adjust exposure a little in post if needed.
Use long exposure noise reduction unless you can't afford the time for an extra exposure. Don't use it with star trails it takes too long.
Star trails are another matter altogether. They require long lenses and long exposures - often an hour or more.