Shooting portraits outdoors. I use primes now and often around f2 to have more of the face in focus while still having a strong background blur. A good f2 zoom would mean I don't have to swap lenses (so much). Not having 24mm is irrelevant to me - if anything I'd rather see this lens to be a 35-80mm/f2 or even 40-90mm/f2. My dream portrait zoom would be the Sigma 50-100/f1.8 but in full frame version.
As for the weight/size - I'm just a hobbyist so I don't shoot for 8 hours straight. I would also never take this lens on a hiking trip. But I mostly shoot portraits and using the Sigma Art primes for the past few years has made my arms strong ;-)
I understand your needs
I used to shoot portraits with fast primes, mostly 50 ish primes so I was using apperture to get subject separation. With time I found that I prefer using focal lenght for subject separation and felt in love with the 180f2.8ED. Now, with the stabilisation, it is just a joy to use (sport stab and EFCS).
Sigma primes are good and, somehow, the S line looks similar. Infact, the S line is very well build, nothing close to the "plastic fantastic kit lenses …" some naysayers forked over YouTube channels (Like the Northrup and Cie).
It is good metal, good plastics (feel like metal) and good rubber. Rings are super stiff, I only wish that "kit lenses" approach this quality ! And, cherry on the cake, the optical performances are mindblowing.
My previous camera is a D700 (I keep it, less than 50000 act since I bought it) and the weight/bulk ratio is full tilt in favor of the Z. Even the balance on my flanc is enjoyable and non intrusive as a D serie.
I love Nikon but I envy some canon lenses, the 135f2L for exemple. I hope Nikon will build Something similar in 2021.