Francis:
Thanks for your comments. I have used the Z6 since it was introduced, and find it to be very good for most of my work, except it is still not as good for Birds in flight as the D500 or D850. It does fine with larger birds such as swans, geese, etc., but struggles with small erratic flyers. I have the latest software, and it has gradually gotten better, just not quite good enough.
I look forward to seeing what the Z9 can do re: tracking.
Keep working on the BIF, some of it is absolutely muscle memory to help track well and move smoothly, learn to anticipate framing, etc.
Cheers
Randy