Having a Z50ii as my only camera I do see 16-50 2.8 as a good choice but 12-28 you mentioned is under consideration even when I had my Z8: a lightweight affordable super wide lens 
On the other hand the 16-50 kit lens is not bad, had it in the past and very satisfied as my only dx lens
Regards
I agree that the 16-50/3.5-6.3 is good enough that the 16-50/2.8 is only an obvious choice if you need the superior optical performance or capacity for subject isolation. That is more likely if, like me, you are DX only, but even then I
need the 16-50/2.8 rather than the 16-50/3.5-6.3 only occasionally. The catch is that for those occasions it is hard to see a cost or weight competitive alternative to the 16-50/2.8.
The 12-28 is much better than its weight of 200g and cost of US$400 would suggest. As Birna points out, it has a lot of barrel distortion at 12mm (-15% radial, according to Bill Claff's Optical Bench site) so you lose some peripheral sharpness when that is corrected, but you have to spend a lot more money to do better (eg, at 14mm the 14-30/4 is about -7% at the DX boundary and the 14-24 is about -2%).
The 18-140 is inferior to the 16-50/3.5-6.3 in the overlapping range, so it is the longer focal lengths that make it attractive.