I can tell you what works best for me. I've had both generations of the Z6&7 and currently have the Z9 as well as the Z6ii and Z7ii. I have had the Z9 out every day since it arrived and have it set up in a way that's great for me but might not work for others.
Function 1 button-AF area mode
Function 2 button-silent shutter
Function 3 button-Full area AF
Function4 button-FX/DX
AF/ON-3D tracking
Movie record button-Metering
Lens function button-AE lock
Shutter focus
This setup works fantastic for nearly everything I've thrown at it. I normally keep in High shutter backed down to 10FPS and very rarely bump it to 15 FPS. I probably have well over a hundred hours with this camera now and think its pretty phenomenal. The only two gripes I have are, the weight and not understanding why they have taken away the ability to assign spot metering to the lens function button.