Rich. The Peak Design 30L Everyday backpack will hold the z9 with the 100-400 attached (but not with hood extended). It will hold the 180-600 or the 600 f6.3 PF lenses, but not attached to camera body.
My plan is to travel by plane with the new Peak Design Roller Pro without the camera cube insert. Cameras and lenses wrapped in lens coat protective sleeves will be in that bag, including the 600mm f4. My laptop will also be int the Roller Pro. As a 2nd carry on bag I will take the Peak Design 30L Everyday Backpack holding any extra camera gear and other personal items. For walking through the airport, I can have the Everyday Backpack on my back or on the handle of the Roller Pro. On the plane, the Everyday Backpack is small enough to fit under the seat if no room in the overhead for the 2nd bag.
Bags will be repacked for field use. When I am in the field, I will carry my 600mm f4 attached to camera body in my hand, on tripod or in Jan Gear lens carrier pouch. Other equipment including camera body attached to 100-400mm lens will be in the Everyday Backpack. I will also carry a 24-120mm and perhaps a 14-24mm, 1.4X TC and any other gear that I need in the field.
When I take the 600mm f4 TC lens on a trip, I really don't need the 600mm f6.3 PF or the 180-600mm. I take those lenses on trips when I am not taking the 600mm f4. And then, instead of using the Peak Design Roller Pro and the Peak Design Everyday Backpack, I will travel with the Gura Gear's 30L Kiboko backpack and Chobe bag. If I feel the fully loaded Kiboko backpack is too heavy to carry as a backpack through long distances in the airport, I might use it with the small Samsonite foldable luggage carrier.