[you2]

Are there examples of lenses that vignet less at these focal lengths on Sony mirrorless FF? If not, maybe it is more a function of the small mount diameter, rather than "underdeveloped design"?

I only checked two lenses but both of them - the 90mmf2.8 and 135mmf1.8 had less vignetting wide open. So the answer to your question is yes there are example of lenses that vignet less at those focal lengths.