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Actually this is Foveon that is providing “rawer” pixels when you think about it. Bayer and Xtrans sensors all interpolate the colors at some point. Sigma is emphasizing this on their communication and there is a bit of truth in it. When you look at the pixels, the sharpness is really impressive. And from my tests, this can make a difference. But you are right, Foveon camera are very slow. But not as slow as you describe. On the dp0 there is a 7 pictures buffer and you can take about 1 picture / s until this is full and then you have to wait a bit (a few seconds) - on average I would say that you can take 1 picture each 2s (which is I know slow by today standards but totally ok when you work on tripod).
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Yes, you can close your aperture lens but remember that fast lenses are heavy (more glass, more metal, more weight…) so you may end up carrying 1kg of glass for the same result than 300g… About the A7 series vs A7R, the “R” have no lowpass filter, and have better resolution (42MP on the A7R2 VS 24MP on the A72). It can make a big difference. And the lenses are the same, this is the FE mount series (full frame). But at the end you should take the camera you are comfortable with. The best camera is the one you know well.
50 mm looks a bit long. You will have a shallower dof so you will have to close more and take more pictures (because you cover less with a 50mm) - but on the other end this is usually very low distorded and sharp, which is good. Personnaly I prefer to stick to wide angles (usually 14 to 35 mm) but this is a matter of choice also.