How is Z6ii sale volume compared to Z6?

I suspect volume of the Z6ii is very good, but part of that is the way the camera is positioned. In my mind its a near perfect standard resolution camera - it does just about everything well.

The Z6 was already popular for a similar reason. Because of the similarity to the Z6 and Z7, I suspect Nikon had more ramp up time with parts and the supply chain than normal. That makes it easier to start with higher volume production. The Z6ii monthly sales will outpace most or all other FX cameras from the start, and can sustain that rate.

I find the Z6ii has several significant advantages. The second processor is a much bigger deal than most people realize. Look at the other top cameras - the D500 and D850. Both of those cameras have a second processor for AF. The Z6ii has a second Expeed processor - not just a second processor. That's a more powerful processor that supports not only AF speed but also image processing and write speed. The frame rate, write speed, and sustained burst rate are more along the lines that Nikon conceived when they designed the Z cameras. I also expect the second processor helps to improve the display rate of the EVF, and that reduces blackout for fast action.

For what you describe, the Z6 would be good, but the Z6ii looks like an ideal camera. The 24-200 lens is a terrific travel lens. If you need anything else, there are some very nice primes and zooms. But the really nice thing for video is you can also use older F-mount lenses - including vintage lenses with manual focus and they perform seamlessly.