I did a lot of investigation of possible lens choices to expand the telephoto/birding capability of the Z8/9. Here is a summary of what I believe to be some of the best choices.
1. Older, cheap exotics: You can find older top quality functioning F mount lenses for under two grand. The drawback is that these lenses tend to be very large and very heavy.
2. 500mm PF: this lens is really highly rated for quality and it is one of the most effective reasonably priced birding lenses in the F mount. They can be found used for roughly $1800.
3. Z 180-600 zoom: New design, amazingly inexpensive. Not an S lens but has had excellent optical reports despite some limitations. Lens is oversubscribed with a long waiting list. Cost about $1600. Lens is relatively large and heavy.
4. Z 100-400 mm zoom:: A well-regarded zoom option in a S lens. Many prefer this as a zoom option working with a longer prime. Most likely above 2 grand but might find some used slightly under.
5. Z 400mm f4.5: A prime quality lens that is optically close in image quality to the reference top quality prime the 400mm f2.8. I have this lens and it is incredibly sharp, will outperform other lenses so far on this list except for the 500mm pf. You can use it effectively with a teleconverter out to about 560 and less so out to 800mm using a 2x tc. Cost about 3 grand but you might catch a used one for a bit less.
6. Used late model F mount super primes including 400mm f2.8 and 600mm f4: These are state of the art top quality prime lenses that cost 14 grand or more. Look around carefully and you will find some of these available in the 5-6 grand range. These are lighter and more current than the older models but significantly heavier and lacking in features in the latest Z super primes.
7. Z 600mm f6.3 vr s: The apparent successor to the F 500 PF this lens is superb optically and has been compared favorably to the super prime 600's. I have one of these, cost is about $5600. Not as fast as the super prime 600. Get most of the optical performance of the $16 grand 600mm f4 at a third of the price. This is a very effective lens with a 1.4tc at 840mm and can be used out to 1200mm if the subject is large in the frame.
8. Z 800mm F6.3 vr s: An amazingly effective lens it is the equal or better in its optical range than anything else out there subject only to the super expensive and super heavy F 800mm f5.6. This lens sells new for $6500 but you can find used ones for $5500. I have one of these. Need to work in conjunction with a shorter lens for anything under 800mm but it will handle anything possible longer.
9. Z 400mm f2.8: Priced at about $14 grand it is the reference lens at this focal length. It can work both with an internal tc and external tc to provide options at various longer focal lengths. Choice between this and the next lens for the ideal birding lens.
10. Z 600mm f4 tc vr: At $16 grand it is the ultimate birding lens for many. Through use of teleconverters will rival the 800mm at all longer lengths. Many prefer this lens as it is both the best 600mm lens and also equal to the 800mm lens.
Makes logical sense with much of your assessment. Enjoyed your views.
The 100-400 is i feel really very much overpriced for what it is.
Older glass, i sold my big 600 F4 a few years ago, mostly due to infrequent use, I do find the 300 2.8 VR II i have offers F2.8, or F4 with a 1.4Tc, invaluable for most of what i do, yes 1.5 kgs heavier than the newer lenses but its F2.8, not F5.6 or F6.3, personally i don't have a strength issue and i throw my 300 2.8 around like a 28-300 LOL, it may not have the reach for many birders, that said, i am but not a major bird shooter, but for so much else F2.8 is a versatile very powerful light gathering tool with stunning background results, i use it 90% of the time at F2.8 therefor with very little ISO needed, the image quality is super tolerable to cropping, this married up to a 45 or if ever available a 60mp sensor is still great for what i do. Usually i will rent if i need anything different.
I think the new Z options are fabulous, especially for weight size and TC tolerance, all these features make them very very attractive, but largely compared to any F2.8 glass makes them a little more fair light fair weather lenses.
My preference is the 400 F4.5, 500 PF or 600 PF but purely for size and weight for travel or lots of walking around. Nikon has done a great job in filling in this range or area, sadly with a little to much price.
For myself sharpness depends on the user, light is the best tool of all, like using a flash, in principal the more light makes more colour, contrast and detail as we know.
In fair conditions most lenses all seem very much the same, sharp and clear, not so in less ideal conditions, dawn dusk, overcast, night times under lights with heavy shadows.
When Nikon comes out with its 300 F2.8 PF it will be much lighter and possibly smaller BUT at a very very expensive price, why so, because can name its price as we always pay LOL.
If you want it you will pay for it, simple, LOL.
Only an opinion
and that is me being $10k or more lighter down the road.