Not sure what you use for software to process images. Recent noise reduction software can do a lot to improve an image if you need to use high ISO, due to low light and to stop subject motion.Thanks for your comments. I appreciate it. We will be in the Haines area. I think I will end up with the 600mm 6.3 and perhaps I'll rent the z100-400mm too. If I don't rent the 100-400mm, I may just end up taking my 70-200mm. I do have the z2.0 TC.
I tend to use DxO Photolab Elite (now version 7) or Pure Raw (version 3, but I see that a new version 4 just came out) to reduce the noise in higher ISO images. It tends to work quite well. Sometimes I use the AI noise reduction that is now part of Adobe Camera Raw and Lightroom. It's pretty good too. Depending on the image (in ways I have not figured out), sometimes one approach can be better than the other. I also find it useful to vary the level of reduction and/or sharpening applied using sliders or settings. I have also used Topaz AI products for noise redction and sharpening.
This type of software has made me more comfortable with higher ISOs, when needed, than I used to be. Shooting a Z9 and Z8 these days.