In your situation, I would go with the D500. Remember that you will need to budget for lenses as well for the upgrade to give you the most benefit. The D500 is just as demanding as the D850 as the sensor pitch is the same.
Do not forget to get fast memory cards from the start as you will need them.
If you plan to shoot much aviation, especially prop aircraft, get a grip for whichever camera you get. Panning is much easier when the camera grip gets a bit taller.