Awesome! I'd never thought of using a variable ND in video like that...
How did you come to hear that the BBC won't accept less than 4K?? They don't broadcast 4K or stream it from iPlayer, so why would they be bothered? Genuinely curious not trying to start a beef
I use both a Sigma 18-35mm and a Rokinon 85mm on my 70D and I do find the Rokinon produces a nicer image. I'm not sure if I'd call it softer necessarily, if anything it seems a little sharper in picking up detail, but it certainly has a much warmer colour; I often hear Sigma's lenses being called a little cold and clinical by comparison.
Of course the Sigma has the advantage of being a '3-in-1' zoom, but then the Rokinons have much nicer focus rings and separate aperture rings too... ultimately, if you have the money for a comprehensive set, and are filming in controlled conditions with time to change lenses regularly, the Rokinons will get you better footage, but it's a trade-off against usability.
I thought it was fantastic!
Perhaps it was because I knew beforehand that this had been made as a showcase for the composer, but I had no problem with the 'story' side of things. I can see why others here struggled in finding a consistent logic to it, but to me it was obvious enough that we were seeing the imaginings of the dancer as she moved with the music, and I thought the twist when she fell from her wheelchair was genuinely impactful and put things in a new perspective. For me, the emotional journey was enough without needing to understand the events literally.
There were a few small things I might have done differently but that's just personal taste really. However there was one thing it might be worth considering: I love how the opening moments don't give you a good look at the dancer's face, it gives a subtle tension that draws the viewer in and fits with the music, but I think her face should stay obscured all the way up to 0:41 when the music hits that big chord. Even on my first viewing, I found it a little jarring seeing her full face on the bridge at 0:34, it doesn't follow the flow of the music to release that tension there. Less noticeable is the focus rack into the mirror at 0:21, but if possible I'd avoid that too. Of course it depends on what other shot options you have for those moments, but I think it would do a lot to strengthen a really nice dynamic that's present in that opening section.
More importantly what did you film it on?? It looks really good and I'm guessing I couldn't nearly afford whatever gear you used but it would be nice to know!
Just to point out, if you're referring to the Sigma Art 18-35mm f/1.8 as the wide angle lens you're looking at for real estate work, that lens is for APS-C size sensors (like an EF-S lens). The A7Sii can film in an APS-C crop mode so it's not useless, but then you'll have about 29-60mm in focal length, so no better than the Canon 24-70 lenses for wide stuff...
Canon do some 16-35mm lenses in their L series if you have the cash, or perhaps a wide prime might be more economic if you're only going below 24mm in certain situations.
Why not get both?
I find it's easiest to get the actors to do it more naturally first, then do a few extra takes where I ask them to keep all their lines nice and separate. That way you have more freedom in editing if the natural takes don't work out.
...of course that does mean the scene will take longer to film, but not every scene will necessarily have characters talking over each other.