Well done ! I have same setup, however have not managed something quite this good.
I see your point & eluded to that in my initial comment. Which stitching program did you use? In AutoPano Giga, you have all sorts of options with projections. I have seen these sorts of pics without the curved Milky Way.What I'd like to do one day is Use a equatorial telescope mount with motor drive to take the Milky Way, so you could use a lower ISO, longer exposure& star movement will not occur. You could edit out edge of field distortion & vignetting and then us the horizon that suits. Huge job however I think it could be worth it.
I really like this pic, however the horizon isn't curved but the Milky Way is.So, it appears the horizon was superimposed. The Milky Way, in reality, will not look curved to the naked eye.I'd rather see a straight Milky Way with that horizon. I suppose it comes down to what lenses you're using for the stitching and how much of the Milky Way, you're trying to include. Seems 3200 ISO is the way to go with full frame & these kinds of pics. You also have a problem with earths rotation unless you have a motor driven equatorial mount. Its all a juggling act, however this still a very nice pic and quite a bit of work must have gone into it.
It’s interesting to note Nikon are opting for a small sensor. I think it might be a brave move. Small sensors up till now mean poorer dynamic range & poor low light performance. Even although Nikon think they are appealing to a new market segment my hunch is that APS-C size sensors in Mirrorless camera’s will win out eventually. I think micro 2/3 sensors will struggle eventually also. At the end of the day we all want lighter & more compact DSLR’s, & even rank amateurs will take note of reviews on the net before buying.