My main lens is the 24-70 2.8 and the 70-200 2.8 ( D700 and D810 body )
For travel lens I bought the 28-300 VRII and it too soft in the corner even stopped down to F8
I sold that and bought a 24-120 F4 VRII ( the VRI version was a dog BTW ) . Quite frankly I'm surprised how well it holds its own against the pro lenses
The reality is you can often get sharper pictures for interior low light with the 24-120 because of the VR ( I take lots of Cathederal and other architectural when traveling in Europe )
I guess it really depends on what type of photography you do . The reality is in many situations, the F4 will suite you fine . For me, I use my 70-200 as portrait lens so I want the 2.8
Center sharpness the 24-120 is surprisingly good .
Again, depending what you are doing, a tiny bit of corner sharpness may not be an issue . For example, subject matter may be in center of frame, or using shallow DOF that your corners will be OOF anyways
What drove me nuts with the 28-300 was the relatively slow focus speed. The 24-120 VRII is pretty darn fast
I think the 24-120 F4 is a very good solution vs 3 primes ( 24 1.8 or 1.4 , 50 1.4 and 85 1.4 etc )
The reality is there is no cost effective prime solution, since all these lenses will cost lost more than the 24-120 !
IMO you will find the high iso capabilities of the D750 good enough that you won't need the fast 50 and 24s for low light . For what I shoot, I want the fast lens for shallow DOF for portraits, so I'm bare minimum 85mm, typically 105 or longer
I think you will be happy enough with the 24-120 VRII long term, and eventually supplement with ultra wide ( 16-35 , 17-35 , 14-24 ) and a good portrait lens like the 105 DC , 135 DC or even the 85 1.4
I think this lens will also work great for product photos since the minimum focus distance / focal length is fine for product photos . Even if you are taking pics of small items you can always zoom in
For most product photography you don't need the 1:1 macro reproduction ratio .
Sorry , but I have no experience with grey market .