While I do plan to do a bigger post on my blog with more detail; since I have all three Z lenses that reach 400mm I thought there would be some value in just noting down their pros and cons and where I find each most useful.
All my opinions are based on the view of a wildlife photographer that used to be a product photographer. I earn a good deal of my income from photography though I daren't say I am the most skilled there is yet (I am working on it!)
All my views are also based on using the Nikon Z9, which I have had for a year and two weeks at the time of writing.
So let's start with the 100-400 and let me say this is probably one of my least used lenses for wildlife despite how versatile it is on paper. I find this the most useful when I am going out to get damselflies and butterflies, I find it too short for squirrels and f/5.6 is too dark for a walkabout lens as the woods near me can get rather dark,
Next up is the 400 f/4.5 which I have to say is just fantastic and a real surprise when it was launched. It turned out I got this lens the same week as the 100-400 which may have poisoned my views on the zoom lens since the prime is just so much better. I use this lens extensively for woodland critters, where the zoom lens far too often results in really noisy images (You have to double the ISO or half the ISO for the zoom, and we are already looking at 1/100s at times for the prime at ISO 8000)
Finally the big boy! The 400 f/2.8 TC is the newest edition to the kit and it is a fat boy yet it somehow is just as handholdable as my 800 PF. My fear for this lens was that it would need to live on a monopod or tripod which would limit my movement and range. Thankfully my fears were unfounded as the 400 f/2.8 TC is well balanced and I've handheld it for entire days with very little fatigue with the caveat that this is when moving about, when stationary it can start to feel heavy and this is when I use my monopod (or tripod if in a hide). I was also worried if 400 was long enough, but then I got used to the built-in TC which has no IQ loss I can perceive and I no longer do I desire the 600 (especially after seeing how many subjects were just right for 400 and didn't need the TC).
So a question I often get is why keep all three (originally each was a stepping stone while waiting on the big lens). And the answer is that these are all very different tools and have very different use cases. I often take the 400 f/4.5 or 100-400 f/4.5-5.6 out with the 800 f/6.3 for instance, I couldn't realistically take both big lenses out with me together on a hike as that is too much weight on my back, but one of the smaller lenses does the job. Or on my daily walk(s), I take the 400 f/4.5 with me just for that odd deer, robin, or fox that pops up.
400 f/2.8 TC off:https://www.flickr.com/photos/thephotobunny/52571450235/
400 f/2.8 TC on:https://www.flickr.com/photos/thephotobunny/52571208192/
What do you use the FN2 button for (apologies if you have already said and I've missed it)?
As for my Fn buttons: Fn1 single point, Fn2 3D tracking, Fn3 full area, lens fn1 DX/FX switch, lens fn2 is wide area small or focus recall (Except on the 400 f/2.8 which has a dedicated ring.)
On the Z9 though I just leave it in VR sport and forget. There has not been an advance to normal VR nor VR off in any shooting circumstance so far.