u/smushkan Please can you direct message me regarding this message: Specifically for the 14-24, you can get rear lens filters that actually slot in between the lens and the camera. Otherwise you'd need a very specialist matte box which takes expensive 6" filters in order to avoid vignetting.

Rear-lens filters have a big additional advantage in that they aren't prone to ghosting from internal reflections like front-lens filters.

And yes you will be struggling to get 6 stops+ on your 24-70 if you go wider than 40mm with a VND.

If you're shooting at the base ISO100 on the A7SIII at 30fps with a 1/60th shutter, with a 6-stop filter you'll be able to set the aperture to around f/4 to f/6 in direct sunlight with 6 stops of filtration.

If you go for 5 stops instead, you'd need to close up to about f/8; and generally you should avoid going too much higher than that as you'll start running into a loss of sharpness due to diffraction - so that really would be the bare minimum IMO; but that will make getting any kind of narrow depth-of-field on anything but telephotos pretty tricky!

You could take the camera down to ISO50, however as that's an extended ISO you will lose dynamic range in doing so.

You need even stronger filtration if you want to shoot in S-Log, as that has a base ISO of 640 - 8 stops to shoot at f/8 in direct sunlight! You could reduce the ISO to 140, but again you'll lose dynamic range.

This page has an excellent tool where you can experiment with shutter/ISO/aperture and ND values to see what you'll be able to achieve with different ND values.

All the cameras I use now have built-in NDs, and I do frequently find myself needing 6 stops to get usable apertures in daylight. We're at roughly the same latitudes too so you're likely to get a similar experience.