I understand your desire and reasons for third party and legacy lenses, you have a point there. I am using to buy mostly Nikon mount lenses, because they are broadly adaptable to the rest of the mounts. However, truth is that today we have larger participation of a “system dedicated lenses” than ever before. Starting from the sensor itself, pixel pitch, microlenses, sensor toppings, etc., many of those specifics are taken into consideration during lens design.
Examples of such approach, are recently Loxia 50/2 and 35/2, which are basically same design as their ZM counterparts, but adjusted (mainly the rear lens group) in order to correct astigmatism on Sony E mount cameras.
On top of that, you were considering “landscape lens” and believe me that at f/5.6 and smaller, you will hardly find differences between Sony FE 16-35/4 G OSS and Otus 28/1.4 (except for the traces of LoCA with Sony).
I am using filters extensively and I certainly don’t want to comment on Mr. Rockwell blog, but with screw-in filters you will face several problems…
LEE is making best neutral filters IMHO, in terms of their “neutral” color behavior. So called Big stopper, does shift colors to the blue tones, but that is way better than BW screw-in 10 stop, which is shifting them toward red.
Using system filters, such as LEE, will allow you to combine 2 or even 3 filters at once, unlike with screw-in. (especially on WA lenses, where stacking filters might cause vignetting).
Most importantly, I am always laughing when I see someone using screw in ND graduated filter on the sunset (sunrise)… Tonal gradient is going through the center of the filter, while one of the most famous composition rules is rule of thirds. Thus if you want to place gradient properly on the horizon, you will ruin your composition.
BTW, for proper exposure balance on the shots where exposure compensation is needed via ND filters – such as sunset or sunrise, you should anyway consider so called reverse grads or strips, and while they exist in screw-in variants, they are even more useless than regular screw in ND grads.
I am using LEE hard ND grads, most often in combination of 0,6 and 0,9, by rotating one of the filters (usually weaker one) in its slot by 180°. At the horizon line, I let both filters to overlap, giving me 1,5 in that zone. This way, I can adjust the width of the strip, needed for the scene and I can of course move it up or down, to please the composition. The only caveat of this approach is that I am loosing approx. 1,5 EV overall, so I have to compensate exposure.
Good luck with your choice anyway, and let me know what you’ll finally decide.