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I had the A7iii in Sea and Sea housing and liked it. Traded the camera for the A7Riii. Both cameras fit in the same housing. I was happy with the iii but made the switch for extra resolution ( 42mp in the Riii) more solid construction, the price difference was only about$500 and I could use the same housing. I shoot macro with Sony’s 90 mm lens and am very happy with the results. I don’t shoot video. I don’t expect to change this setup anytime soon. That being said, if I were buying new, I would probably go for the A7Riv. The reviews sound great. Increased mp, improved ergonomics etc. AF might be improved, but I find the AF to be great on the earlier models, especially the iii. Possible concerns are price of the newest model versus great camera that’s been out for a few years, processing the very large files and price of the housing and ports, etc.
These are all great cameras. You can’t make a wrong decision on any of them.