Lots of great advice here. I can only put across what I do and you can judge whether it suits your needs. I have the following lenses: Z 100-400, Z 180-600, Z 600 PF, and Z 600 f4 TC. I used to own the Z 800 f6.3PF but it became redundant when I got the Z 600 f4 TC. I shoot birds mainly and many of these can be small some tiny in fact. l also do landscapes, interesting street scenes and animals but here in Australia animals are mainly only Kangaroos, Wallabies, Koalas and Wombats. Kangaroos etc don't do much that other than hop, the rest of the time they are fairly boring, as are Koalas and Wombats etc, thus animal photography here is limited in its interest.
So, with that in mind, birds are the majority of my photography and with birds, 600mm is the minimum and I am often at 840mm. My Z 600 f4 TC is my go to lens. I have used the 180-600 and actually rate it highly, much better than it's price point would indicate. The Z 600 pf is also a great lens and I often use it with the 1.4x TC and it is still sharp as a tack. I rarely ever use the 100-400 as I find it too short for my birding it is an excellent zoo lens, though.
I can post sample images from all lenses if you are interested, or you can go to my individual galleries on PBase. When you click on the image, underneath is options for image size. Best option is "Auto" as it sizes the image to suit your screen:
