I have the Z 100-400 and like it a lot on my Z9. Expect to like it on my new Z8 too, but have not tried it yet.
I went to Katmai NP to photograph brown bears during a sockeye run in July/August 2021. At that point, I did not have my Z 100-400, so I used a Z 70-200 with either the Z 1.4x TC or Z 2X TC on a Z7II and a 500 mm PF on either the Z7II or a D850. I thought the Z 70-200 with Z TCs was good and the zoom flexibility was often very useful.
I got the Z 100-400 in December and have used it on a number of photo trips, including two trips focused on bears — the Khutzeymateen (grizzly bears, May 2022) and Hudson Bay (polar bears, November 2022). I think it is a very good lens if you need zoom flexibility or if you need a close minimum focus distance. In both the Khutzeymateen and Hudson Bay, we were often close to bears and there were also times when photographing the bear with its environment was nice. So I appreciated the short end of the zoom. And there were times when I wanted 400 mm or longer (especially in Hudson Bay).
I think the Z 100-400 is better than the Z 70-200 with TCs. It’s also better than either of the versions of the F 80-400 that I had in the past.
If you know you will not need the shorter focal lengths, the the Z telephoto primes are certainly better optically. I also have the Z 400 mm f4.5 and the Z 800 mm PF.
I also think the Z 100-400 works quite well with the Z 1.4x TC, getting you to 560 mm f8 at the long end. If you can live with f8. I also often have two bodies set up with me. At this point, that is often the Z 100-400 on one body and a longer lens, say the 500 mm PF or 800 mm PF, on another body.
One other point, I find that with bears, depending on circumstances, you may want to stop down some to get more of the bear in focus. For example, avoiding sharp eyes with blurry nose and ears.Grea