I used the Z 70-200 with the Z 1.4x and Z 2x TCs before I got the Z 100-400 mm lens. Here is a link to a SmugMug album that has some shots that I took with those two combinations on a Z7II (and captions identify the combination and focal length used) of brown bears in Katmai National Park in 2021.
This gallery hosted by SmugMug; your photos look better here.
wwelke.smugmug.com
I think both combinations work well — shots with the 1.4x TC are a bit better than the 2x TC at the same focal lengths. You may end up with a bit more sharpening needed. I generally used Topaz DeNoise AI and/or Sharpen AI on these photos.
I think the Z 100-400 mm lens is bit sharper at equivalent focal lengths than the Z 70-200 with the Z 2x TC, so I generally use Z 100-400 now on wildlife outings. Last year, I used the Z 100-400 on trips to Yellowstone, the Khutzeymateen, Grimsey Island and Hudson Bay. (It also works well with the Z 1.4x TC, giving you 560 mm f8 at the long end.)
I was on a raft trip in the Grand Canyon in April. The trip was focused on landscape photography, so I had the Z 70-200 along. I decided to throw the Z 2x TC in my bag in case we came across some wildlife I wanted to photograph, since it was easier to add the Z 2x TC than take both the Z 70-200 mm lens and the Z 100-400 mm lens. Ended up with some nice lizard photographs using the Z 70-200 and the Z 2x TC.
Both Thom Hogan and Brad Hill have comments about these options on their websites that you might find useful.