Samuel H
Major Contributor
I would vote against the BlackMagic 4K sensor. I don't think you're going to like the low dynamic range on that one. The 4.6K should be more than fine, but it's out of your budget.
And since you're already shooting Sony and don't mind a non-video-camera body, I would say, one of these:
* a7S II (it shoots 4K slog2 with slog3.cine color; the internal codec is good but not great) (it only shoots 4K in full frame mode, which means you need new glass)
* a7R II (very similar to the a7S II but without slog3.cine color; it shoots top-notch stills, no idea if you have a use for that) (it shoots great 4K in APS-C mode, you can keep using your glass; rolling shutter is very bad in 4K APS-C, though)
* a7S with external recorder (shoots 4K slog2 but without slog3.cine color) (it only shoots good 4K in full frame mode, which means you need new glass)
And since you're already shooting Sony and don't mind a non-video-camera body, I would say, one of these:
* a7S II (it shoots 4K slog2 with slog3.cine color; the internal codec is good but not great) (it only shoots 4K in full frame mode, which means you need new glass)
* a7R II (very similar to the a7S II but without slog3.cine color; it shoots top-notch stills, no idea if you have a use for that) (it shoots great 4K in APS-C mode, you can keep using your glass; rolling shutter is very bad in 4K APS-C, though)
* a7S with external recorder (shoots 4K slog2 but without slog3.cine color) (it only shoots good 4K in full frame mode, which means you need new glass)