I suspect you would love the 16-35mm II for landscapes....my son-in-law has one and I have used it and it is very nice. Landscapes look great to me but I am not a pixel peeper. I have read that the coma effect is a problem for astro-photography but I have no experience there. I have used it for indoor shots at family events too....it functions quite well as a general UWA zoom but not as useful as the 24-70mm f2.8 in that regard (I understand you are not considering it now but that focal length range is really nice and it goes so well with your 70-200 too)

I do own a 17-40 and a 35 f/1.4 both of which I really like. I have no complaints with the 17-40 on landscapes but I am sure the 16-35 has slightly better IQ especially away from center. I bought the 17-40 because of the cost differential and I did not think I needed f/2.8 For indoor/lower light events I crank up my 1DX ISO or add some fill flash. Currently I do not do any astrophotography.

I think the Canon 24mm f/1.4 or 14mm f/2.8 or Zeiss 21mm f/2.8 would be the best (and most expensive) for starscapes as the 35 is not wide enough.

You could try some stitched-panos with your 70-200 too, you might be pleasantly surprised.