Some interesting discussions on lens choices .... I normally just have these discussions with myself, since these types of choices are naturally very specific to one's circumstances. But I'm curious to get some additional thoughts to swirl around in my head with my own thoughts.
Currently I shoot a significant number of my images on the really long end of things. Almost 50% of my shots are from 800mm+ and that percentage is higher if you start from the time I got the 800f5.6. Typically I'm either shooting small subjects, or I'm in a location where it's physically not possible to get closer to the subjects (one step closer and you fall off the cliff, or fall off the pier, or get submerged in water, or you're not allowed (legally and/or ethically) to leave the trail, etc). I absolutely love my 800f5.6 and love it even more with the Z9. I used to rarely use a TC other than the factory-matched 1.25X. Now I regularly use the 1.4 and sometimes even the 1.7. Of course hiking miles up-and-down steep inclines with the 800, plus monopod and other lenses is pretty fatiguing especially with some of my health issues. So when the 800PF was announced I ordered within hours of pre-orders opening. I'm still anxiously awaiting its arrival (non-NPS). Though I am curious to see how well the 800PF compares to the 800f5.6, especially in terms of backgrounds, particularly with specular highlights.
One of the things I'm trying to figure out now is what other lenses should be on my road-map, especially considering that hopefully soon I'll start traveling a little more (haven't flown since pre-COVID). Currently my travel, such as it's been, has been car-based so I can "pack heavy", though I still get grumbles from the family when half the trunk is filled with camera gear before we even start trying to put in suit cases and other family-related things.
I already have the Z f2.8 "trinity" which gets everything below 400mm OK. I also have the 120-300f2.8 F mount, which I absolutely love, but even though it's "only" 300mm, it's a fairly large lens and I don't think I can really manage that plus something that gets me out in the 800 range. Maybe the 400f4.5 plus TCs, but I've gotten pretty addicted to being able to shoot in fairly low light. That's why the 400TC seemed like a nearly ideal choice. The trinity (plus TCs) cover the <400mm range reasonably well, and the 400TC plus 2X TC would cover the 400-800 range pretty well. Or maybe wait for the 200-600 ... though Nikon's marketing has done a pretty good job convincing me I really want/need the S-line lenses ....