Personally I think the best range for FX sensors is between 24-36mp (D750-D810), larger than that and it really starts stressing many of the lens' on the market.
Might do a little testing tonight and see how it gets. 1s exposures at ISO1600 should be all that's needed at F/1.4
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DXO measures lens performances. Lenses always score higher with higher MP sensors like A7r2 and Canon 5DSR than the same lens with a lower MP sensor. Its true you start to see some deficiencies with some lenses with higher MP sensors but night sky images are not as high res as a daytime landscape shot. So its not quite true that lower MP cameras match most lenses better. Its more true that you will get more res out of the same lens with the higher MP sensor. My Sony A7r2 with 42mp is better than the D800e and the earlier A7r (same 36mp sensor) in resolution (but not really in noise performance, perhaps a tad).
I would love to see your Sigma Art 85mm wide open at F1.4 to see how that works out. I am currently interested in the Sigma Art 14mm F1.8 but its expensive and as pointed out 14mm tends to curve the image a lot and put strain on the stitching software leaving elongated stars above the MW bow very often.
21mm does not do that.
Greg