Hello @heckflosse,
For your use-case I would also consider a Pentax-K1. You are working under well controlled conditions where you really can get an advantage by using pixelshift.
SURE! 
I have pondered about this choice quite a lot indeed since this Pentax reflex is cheaper than the D850 and I have read really good reviews.
In the end I have opted for the Nikon D850 for these “questionable” personal reasons:
- I am a Nikon guy (just joking of course…);
- We have got plenty of old Nikon lenses in our laboratory;
- Nikon D850 Raw files and features are usually better handled with commercial softwares (e.g. Capture one as regards the tethering implementation). With RawTherapee, I hope it will play along fine (as the Nikon D700 did superbly).
- there are tons of tutorials on YouTube for the Nikon D850;
- I am going to use the Nikon D850 always on a tripod in our laboratory and I don’t mind it is not internally stabilized as regards its sensor (the Pentax has this nice feature instead).
But again, it is a very tough decision since they are both incredible and amazing reflex!