I'm working on a bunch of sharpness tests. I'm going to use a Z7, which is the highest resolving body I own, and will look at diffraction, shutter shock, stabilization and, yes, lens quality (among other things). I want to hold the body constant (other than a cameo appearance by a borrowed GFX 100 to look at the effect of a lot more pixels).
Presently beating the bushes for the lousiest Nikkor I can buy or rent cheap! An early 18-55, some execrable 18-200 or beyond travel lens (yes, I know the 18-55 is a DX lens, as are most of the travel lenses - the Z7 has a pretty darned good DX crop mode), and the first, poor quality variable aperture version of the 24-120 are all suggestions I've received...
I considered that Sony 16-50 power zoom lens on an adapter (it has the dubious honor of being one of the worst lenses in DxOMark's database, and the very worst that isn't either:
On a much smaller sensor (Nikon 1 lenses, built-in lenses on compact cameras, the worst Micro 4/3 lenses, most of which are also 10x zooms)
A 10x or greater zoom
A very inexpensive telephoto
I ended up not wanting to deal with a 3rd party adapter, especially with a power zoom lens.
I have the very sharp 50mm f1.8 Z (and other things - all the Z lenses are darn sharp) for a sharp lens.
I don't have an A6400 around, and it's harder to test with a whole bunch of variables at once. I've used that 16-50 occasionally, and it's terrible (it's notably worse than other "kit" 18-55s). I do have an X-T2 with the 18-55 f2.8-4, and that's a very nice little lens.
Dan