Norm,
What F-mount lenses do you own? I too own a bunch (35/1.8, 58/1.4, 85/1.8, 70-200/2.8, 70-300, 300/2.8, etc.), and don't have any issue using them on the Z. The 35/58/85 I own balance extremely well on the Z, and work great. To be honest, I don't have a huge desire to "fully transition" all of my F glass to Z glass, because I still shoot a lot with my D500.
The only Z lens I own is the 24-70/4, and I will likely get the 14-30/4 as well. However, at this time I don't plan to get any of the prime lenses, nor any of the f/2.8 zooms that will be released. F-mount glass works very well on the Z cameras.
Very good point, I just ran into a picture with the Sigma 35 attached to the FTZ and thought the exact same thing, 'man, that looks unbalanced.' I have a fair amount of F-mount lenses and it would require some 'transition pain' to move to the Z. That is why I've been wringing my hands and contemplating. Then again, I am heavily invested in Leica, µ43, and to a lesser extent Fuji, so what is yet another system?

Cheers. -Norm
The Sigma ART 35 will work fine on the the Z6/7, but the balance will be very different that on a DSLR.