Well Nate, I am not an astro specialist but I dabble a bit, so here are my comments FWIW.
You might find this article interesting: Common Obstacles in Night Photography (cambridgeincolour.com)
I think you mean your RF100-400 lens has the widest aperture of f/5.6. 🙂 I would suggest shooting in manual mode and experimenting each time with the light to get a decent exposure for the conditions - the light changes a lot due to all sorts of criteria, including phases of the moon, and often the challenges are environmental - between light and air pollution you can be severely challenged unless you can find a nice remote spot without street lights and cars.
I would normally expect to see folks shooting star scapes using a wide to super wide angle lens. My pick would be something like the RF 14-35 L f/4 or the cheaper RF 15-30mm f/4.5-6.3 IS STM lens. Conventional wisdom suggests using a sturdy tripod and you likely don't want exposures longer than 10sec, or you will suffer the results of the rotation of the earth.
On the other hand normally the moon would be shot with a telephoto, and that is a very bright object, so you can use your RF100-400 on that hand-held with no issues. Even at 400mm you will be challenged to fill the frame.
cheers, TREVOR
Before you ask us, have you looked in the manual or on the Canon Support Site?
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
