I have an a9ll and find the Tracking works quite well (also have an a1). If you can, try an a9/a9ll with the 100-400; I think you will be impressed with the results. The downside of the a9 is the view through the viewfinder looks less clear (more like an old television) than a D500, or any DSLR, and the buttons on the a9 are a bit small (buttons were improved on the a9ll).