I have never had an issue with uhs-II overheating or speed issues with the a7riii. The pixel shift is 4 captures at minimum 0.25 apart. You can make it longer, but this is the lowest time you can set.
The a7riii and the d850 are both quite close in image quality. At base ISO the d850 wins out because the base ISO is 64. The A7RIII is better at higher ISO’s though.
The price is also a consideration. Sony’s bodies are cheaper and more bang for the buck, but the glass is more expensive generally. What I have heard time and time again, and after years know to be true, is buy the glass first and then the body to match. Your results will depend on the lens more than any other factor.
You should also know Sony makes the Nikon sensor. They make almost all sensors from the Iphone to the Phantom. Canon is the only real competition in this department.