Been shooting landscapes and astro for many years now. I own(ed) the F 20 f1.8G and also own the Z 14-30 f4. I had all three of these lens at the same time and exhaustively tested them against each other with test images and real life situations. In few words, the Z 20 f1.8 is out of this world.
Many people consider the F 20 f1.8G to be the best ultra wide zoom or prime Nikon has ever made, and I certainly agreed with that. That lens is godlike. The Z 20 f1.8 takes things to a whole other level. Center sharpness is a bit better, but really its the mid and corner sharpness that is significantly improved. Crazy sharp corner to corner. Coma is at least a whole stop better than the F 20 f1.8G, and I feel totally comfortable at f2.8 with the Z whereas the F needed to be stopped down to at least f4 before it was mostly controlled. CA is the thing of the past. The only thing the F does better is sun stars, but that can be subjective. Weather sealing on the Z, nope on the F. AF is better and faster on the Z. There is some focus breathing, which I have not seen on any other Z lens thus far.
Vs the Z 14-30 f4 the Z 20 f1.8 is better in every way except convenience. At 20mm, the Z 20 f1.8 is sharper in the center, mid, corners, less distortion, less astigmatism, less coma (compared at f4), better, better, better, and at all apertures. The Z20 f1.8 is for when you want maximum optical quality and/or speed, and the Z 14-30 f4 is for everything else. Its what you wished the 14-24 f2.8's were, but sadly are not. The fact of the matter is that you will never need maximum optical quality and speed and versatility all at the same time. Well, if you know what you are doing. For daytime hikes, the Z 14-30 f4 comes with. For astro, bust out the Z 20 f1.8. Been there done that on the f2.8 "pro" zooms. I would rather have an f4 zoom for general convenience and a f1.8 prime for IQ/speed to fill my all my wants needs in this focal range, rather than live with the long list of compromises of the "pro" f2.8 zoom.
There is a new order to Nikon's best ultra wide's. The Z 20 f1.8 is king for now. The Z 14-30 f4 is #2. The newly released Z 14-24 f2.8 will probably slightly edge out the F 20 f1.8G for #3. The G is then #4. After that is old news.
The Z 20 f1.8 is pricey, no doubt. Several hundred dollars more than the G. Is it worth it? That is up to you. For me, its worth every penny. This is now hands down Nikon's best astro lens with no serious contender. It is almost as sharp as the Z 50 f1.8, which is saying a lot. Weather sealing alone is worth a couple hundred bucks. IMO, this is a fantastic buy if you are serious about landscapes and astro.