Basic requirement button wise would be to have a single point or small area AF with tracking under the shutter release and an Auto-area or 3D tracking under the AF-ON button like my D500 could do but my Z6 can’t, this option seems to have returned on the Z9 however.
The AF functionality is not very Nikon like imho on the Z6, to me the single point AF with subject tracking is the most useful AF area for the slow objects but to be able to use it the camera must be in the Auto-area AF which is the most unreliable AF mode on the Z6 and I have to press a Fn button to access the subject tracking mode each time after the camera is switched On.
I hope I am doing something wrong as I can’t figure out why this was designed as such as subject tracking is the most useful AF feature of the mirrorless cameras over a DSLR.