From your list I like both Oliver and Jack, but my favourites would be Max, Rhett, Finn and August.
I think Max or Finn work best with Sophie and Violet; Finn because it continues the similar and subtle 'i' theme in the names; Max just because it works somehow.
On the topic of Rowan, I personally hope it's classed as a girls name (It's my name, it was a girls name when I was born!), I know it's more popular for boys in the US than for girls but I'm not sure about anywhere else since I'm just going on nameberry stats.
I wouldn't go with Charlie as it has a similar ending sound to Sophie which leaves Violet as the odd one out and I think Zane stands out a bit in the list as all your other names (Inc. Sophie and Violet) are quite classic names and Zane feels more modern to me.
I love Oliver with Sophie and Violet. Jack goes really well to but I understand your concerns with popularity. I saw the suggestion Oliver Jack and I thought that was adorable and perfect since it uses both names you guys love. Maybe it's a compromise?
As for Rowan, I'd say its more of a girl name. And if it isn't fully a girl name yet I think it will be soon so I'd steer clear of it. I know someone named Roland which is similar to Rowan if u like that.
We have quite similar styles I'd say!
From your list i like pretty much all of them and they're all names i've suggested and had vetoed in most cases, so i feel your pain! My fave is probably Rowan or Asher (2 of my faves from my own list!). To me, Rowan is very masculine and I really couldn't imagine using it on a little girl. Rowan Jack or Asher Jack are both very very handsome :)
Thank you so much everyone. All of your replies have been really helpful. I am ruling out Zane completely and probably Finn, Charlie, Rhett and August too. We have 10 weeks left to decide- I'm sure I'll be back here posting a few more times until we settle on something for sure :) Thanks again for all the help!