Welcome to CR forums.
I agree that f/2.8 is very handy for night time photography.... and so the Canon 16-35mm L would be a good consideration. Not tack sharp wide open, but still reasonably sharp - but you might need the f/2.8. Do you need wider than the 24mm? And do you have the 24-70mm v1 or v2?
Other options - yes the Canon 14mm L or one of the TSE lens. Or Canon 17-40mm which is very good stopped down. I don't do much astrophotography, so my criteria are different (I do more daylight landscape photography UWA).
Your Sigma 8-16mm won't work on a FF. For what it's worth, I've seen a lot (!) of very good images with a crop sensor (APS-C) using many UWAs - eg Sigma 8-16mm, Sigma 10-20mm, Tokina 11-16 (f/2.8!), Tokina 12-24mm and Canon's 10-22mm. We're really spoiled for choice!
Then, on the other hand, perhaps you want to consider a telezoom- eg 70-200mm f/2.8 II or the 70-300mm L (which I have). I also take quite a lot of landscape shots with these (eg zooming into mountains, along coastlines, even forests, etc).