The 100-400 is definitely something I would be interested in unless there is a less expensive option. If I do switch, I would probably buy one tele for wildlife and one wide angle for landscapes and use the fuji for my street and the middle options for now. Any other options for tele that I should consider? What about a great landscape lens? I use primes with fuji but I am considering going to zooms both to travel with less gear and have more options, assuming quality and speed doesn't degrade too much over primes. I can also save some money by going with a MF option from Samyang or something similar as I never us AF for landscapes, unless a Sony Zoom would double as a good travel lens and landscape, which should save me money and space when traveling.
In terms of the 3 camera models, if I decide to switch it looks like the a9 and a7III have better AF but the a7riii would have better image quality when blowing up images. Would that be the short version of it?
Sigma 16 1.4 is a wonderful moderate wide angle; I use mine on my A6500 all the time. It is very fast f1.4 so works well for interior available light photography. It has become one of the most purchased lenses for the Sony cropped sensor line.
This was the first native lens I purchased for my A7R. I still have it! I like everything about it (now that it's paid for)...it's small size, Zeiss rendering qualities, sharpness, and nice handling. I did loose the minute lens hood that came with it, but found an aftermarket metal ventilated (not solid) hood that works great. So easy to put in your pocket if not on camera. Here's a shot I took during a round of golf at We-Ko-Pa golf course in Fountain Hills, AZ