I also have shaker cabinets and I have VERY unlevel ceilings (1930s home). Using crown (had I wanted it) would have made the ceiling issue extremely obvious, because the carpenter would have had to trim the top edge of the crown significantly in certain areas, and there would have been gaps in others. Instead we did a flat trim board above the cabinets (similar to but less chunky than yours), and then did a piece of quarter round all the way around to keep the trim from looking too two dimensional. Here are some photos so you can see what I mean.
I do agree with others that crown is not the ideal look for shaker cabinets anyway, which call for extremely simple lines. Whatever you decide, keep it as clean and simple as possible.
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Too much crown overhang
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