Which 35mm 1.4 has the "flattest" focus field at close distance?

I'm only familiar with Canon products and 3rd party support, but if I were answering you in the Canon forum I'd tell you to look at macro lenses which are designed for exactly what you want, for example the Canon 50 f2.8 macro lens.  I'm sure there is Nikon or 3rd party lens that 50 or wider and macro, that will be your best solution.

People who shoot in museums and flat art work use macro lenses for the flat field of view.

--
Hind sight is always better than foresight, except for lost opportunity costs.