The 100-400mm f/4.5-5.6 is a classic design used by countless creatives for genres like sports and wildlife photography, where its versatile focal range and modest aperture offer a lot of reach at far more reasonable prices than top-shelf supertelephoto prime lenses. For Nikon mirrorless shooters, there is the NIKKOR Z 100-400mm f/4.5-5.6 VR S, and this great video review takes a look at the sort of performance and image quality you can expect from it in practice.
Coming to you from Christopher Frost, this great video review takes a look at the Nikon NIKKOR Z 100-400mm f/4.5-5.6 VR S lens. The 100-400mm f/4.5-5.6 takes a classic design and adds a few upgrades and new features mostly aimed toward faster autofocus and improved image quality, and while it is on the pricier side of such lenses, it is still only about the quarter the price one would pay for something like a 400mm f/2.8 lens. While the maximum aperture range is a bit more modest than an f/2.8 or f/4 option, it is worth remembering that the high-ISO capabilities of modern bodies have reduced the impact this might have on your images, and you might find you prefer the versatility of a zoom anyway. Check out the video for Frost's full thoughts on the lens.
I have a copy of this lens. It focuses reasonably fast with a Z7ii, but I suspect it might be better on a Z9. It is somewhat heavy, but not when compared to the f-mount lens options. If 400mm is primarily what you care about, you should probably look at the 400mm f/4.5 for about $550 more.