The noise reduction feature in cameras are garbage, waste of your time running two exposures and also wastes your battery, youre better off using the GIMP noise reduction. But you never know, you may like the way it comes out on your specific camera. just my opinion. I barely have enough patience to run a 30 second exposure let alone twice lol
i wouldnt go past 1600 on your camera for sure, but id still play with it. And if you are serious about getting better shots, throw that kit lens off the cliff LOL. kit lenses are literally the worst lenses for night shots. You should aim for something that goes down to f/2.8 atleaset. maybe a wide angle.
Also , the infinity mark on the lens isnt precise when you focus it on the stars, it will actually be off, so moving it yourself to the infinity mark would do nothing basically. you always want to shoot and zoom into your photo to check aswell. once you feel its sharp lock that bad boy into manual and keep it there.