Claire,
Good timing with your question as this past Friday I received my Z TC-14X to pair with the 100-400mm lens I've had a for a number of months. My lens options for my Z9 are basically complete now, though I'll see what I think of the 200-600 or 600 when they arrive. I had an 800mm PF on order but canceled it due to minimal need...that decision has been confirmed as a good one with the preliminary testing I've done with the 100-400mm + TC + DX crop mode. I also own a 500pf + TC14EIII that will get some use, but even if I didn't have that, I'd go with the same kit.
Incredible that with 1 body, 3 lenses and a TC I can cover from 14mm to 840mm (out to 1050 and more if I include the 500pf + TC) and have IQ superior to anything we could have imagined a decade ago. Granted the 3 lenses are all F4 or slower, but with modern bodies that's not the penalty it used to be, other than DOF/Bokeh...and I have an assortment of F mount primes that will get me that, if I need it. The 400 f4.5 doesn't check any boxes for me and I'm not sure either the upcoming 200-600 or 600 will have use cases that cause me to pull the trigger. The big question looming is what I get for a second body...I miss having both a D850 and D500. While the Z9, for me, replaces and handily improves upon both, it's only one body...arrrrgggghhhhh!
For the wide variety of subjects and genre's I shoot, I will not give up the flexibility of the zoom for that last small % of IQ. I haven't had time to get out and do any detailed testing in a wide variety of scenarios, but I did some quick shots at my home as one of my feeders was being overrun by some juvenile cardinals yesterday. Very pleased with what I was able to accomplish with this "worst case" scenario
This shot was taken handheld with an effective focal length of 820mm...lens at 390mm, 1.4 TC and 1.5 DX crop and then cropped about 10% in post. I was shooting from inside my kitchen, through an open window so having the zoom allowed the best position to compose as I needed and cover a lot of the branch areas of the tree near one of my feeders. As I'm sure you're aware, same situations often apply in the field if in a blind or have limited locations to shoot from. I did edit the image minimally in PL5, though the OOC standard jpg is not all that far off.
Cheers!