Currently I have a 200-500 on a D850. The rig takes great pictures (wildlife, birds, and such, but just turned 80; it is starting to become a beast to lug around.
I have a Z7ii with several shorter lenses, but am seriously thinking about going all mirrorless. So, I would appreciate personal experiences with the 100-400 (with and without the 1.4 converter) and/or the 180-600. I am aware of the speed, size, weight — and cost differences.

Also, I try to have to do as little post as possible — not much could be done with E-6 or Kodachrome for all those years and that’s what I grew up with.

Thanks to all in advance. I appreciate your responses.

As someone who has used both the 200-500 and the 180-600 I don’t even think it’s close. The 200-500 is a good lens and it worked better on the Z9 with the adapter than on my D500. That being said, the 180-600 is better in almost every way. It focuses faster, it’s sharper, it’s weathersealed, it has internal zoom and focus. The only edge for the 200-500 is a constant f/5.6, but I’ll go an extra 1/3 stop for more range and all the other improvements. I actually preferred the 100-400 Z with the 1.4 TC at f/9 over the 200-500.