I have a Canon, albeit the lowest end, old and dinged. Anything higher than 800 is a no-no. Anything 400-800 is only tolerable. Your camera is likely magnitudes better. High ISO images are hard to repair, esp. in your case where you have to worry about not decimating your subjects: the stars and comet.
My suggestion is to aim to photograph the night sky at a lower-than-you-are-comfortable ISO and then boost the signal in post. Perhaps take multiple frames to see where your camera’s and lens’ sweet spots are in terms of settings. A good tripod and locking the mirror may help, though the latter may cause problems when you are using a tripod at the same time (for certain cameras)…
Good luck on your next attempt!
Ah, the skies have been overcast for so long in my area. I haven’t caught a glimpse of the comet yet!