Dave, I don't to astrophotography, but I do a modest amount of night photography and have shot with a number of long-time expert night photographers. I think you may be on the wrong track.
Most night photography is done at base ISO, or close to it, with long exposures. For that sort of work, the low light does not convey any real advantage to FF. The main advantage of FF for photography of this sort is that digital long exposures are limited by heat build-up, and for a given number of pixels, the larger sensor will stay cool longer. This is not an issue for urban night photography, but it is for wilderness night photography, where exposures can be very long. Some people who do that sort of work still use film for that reason.
I'll post a few images as examples. All of these were shot raw with no noise reduction in processing, but the really long exposures with the 50D were shot with Canon's subtractive long-exposure reduction, which subtracts fixed noise and doesn't affect image sharpness.
This one is the only night image I have ever taken that won any sort of award. The competition was not specifically night photography, but the judge was an outstanding professional night photographer. This is a 10-minute exposure at ISO 200 with a Canon 50D, which is not only a crop sensor camera, but a fairly noisy one. I mention the prize not to brag but to make the point that my best success in this sort of photography was done without FF. The yellowish glow, by the way, is not from a setting sun; this was taken around 11 PM. The judge, being a night photographer, knew immediately what it was: sodium vapor lamps. He didn't know this, but there is a tiny village at the other end of the lake, perhaps 6 miles as the crow flies. The village has a few sodium vapor street lights. This is the reflection of that light on clouds. With a 10 minute exposure, it becomes very visible.
Here is another with the 50D, this at ISO 100 and 30 seconds, which is more typical of urban night photography:
I do prefer FF for night photography, and I routinely use a 5DIII now for that, but my point is that for a lot of night photography, a crop is just fine unless you have other reasons for FF, such as printing very large or cropping severely. Just for fun, here is the same lake as the first one, turned 90 degrees, taken near midnight under a harvest moon. 5DIII, ISO 100, 6 minutes:
I think this is an overstatement. I know many people who have done superb landscape work with crop sensor cameras. If I am not mistaken, Donald, who does breathtaking landscapes, used a 40D until not that long ago. Yes, all other things being equal, FF handles low light a couple of stops better. But with really proper exposure--much easier to manage in landscapes than when shooting something that is moving--you can get pretty good results even with a crop. Here is a test shot I took recently with a 7D generation 1, which is not considered particularly low-noise, at ISO 3200, again with no noise reduction at all: