This indicates to me that you have a hot inch burning to spend 3500 dollars on a new camera. If that is the case I would suggest that you trade in your Z7, you might be able to get 1000 dollars for it. Which will drop your out of pocket to around 2500 bucks.
The downside to the Z8 is that it's 200 grams heavier than your Z7, chock full of capabilities you may never use, and it's a bit expensive. BTW, if they made a Z8 200 grams lighter I would be all over that provided it fit in my camera bag. Note, it took me 3 bags to find one that fit my current rig so I am sort of "bagged out". Truth is the Z8 is a fantastic camera for the money.
However too many people turn their noses up at the Zf and Z6 III thatI have to say they are borderline idiots. If you insist on Right Handing all of your cameras Small Rig offers at excellent grip for it that is sometimes bundled with the Zf on the Nikon Web Store.
Note, in over 50 years of doing Photography I have watched a lot of Right Handers drop their very expensive cameras and lenses because they weren't paying attention and the camera slipped out of their grasp. I happen to consider Right Handing a camera as foolish. Use the left hand for the lens and the right hand on the camera controls and you do not need a grip. Add a neck or wrist strap to that technique and you won't hear the sound of a camera and lens hitting concrete.
Monte,
I disagree with this statement about not needing a grip for a variety of reasons. A grip allows for MORE than simply providing something more for "Right Handing" (whatever THAT means!) a camera. It provides more leverage for larger, heavier lenses, which makes it easier to handle those lenses. It provides another set of controls--including shutter release--for use when in portrait orientation, and that helps one with a more stable position when shooting portrait orientation instead of having one's elbow sticking out because of the position of the arm when shooting that way. And then there's the extra battery involved, as well, and for some of Nikon's cameras, the use of the larger EN-EL18 battery, as well.
As for using the left hand for the lens and the right for the controls, I don't know how else one would hold a camera since that's the way they're designed in the first place.
Pluses for the Zf or Z6 III.
First is that 24mp can yield images that print just as well as a 46mp image. Truth is that in order to actually SEE a difference between a 24mp and 46mp image requires prints coming off a 36 inch or larger printer. As for electronic display's, a 4K monitor only yields 8.3mp of image resolution. Think about that next time your posting about how 100mp is a must have. BTW the Zf will do 96mp with the Pixel Shift option and I believe that is true for the Z6 III as well.
Yes, 24mp cameras can yield very good prints, up to a certain size, and probably large enough for the vast majority of people. But to say that the ONLY way to see a difference between an image taken with a 24mp camera and one taken with a 45mp image is from a really large print is mistaken. A larger resolution image is going to have more detail than one with less resolution, and that's something that can be seen in smaller prints. How MUCH of a difference, and if it's enough of a difference for the user, will be up to that user.
Second is the options available on how you actually use the camera if you chose the Zf. Turn off the Auto ISO and setting the ISO is a turn of that dial. No buttons to push, no display to see the setting, you simply set the dial to the ISO. That is efficient and also a bit faster. Actually it can be a lot faster if you preset your ISO for the shooting conditions you are approaching. Another odd thing that happens when the Auto ISO is off is that the Aperture mode actually words as it should, with the Aperture selected resulting in a specific shutter speed. Basically drop the Auto ISO and you have a camera that works like a film camera and you gain full control of the lens aperture, shutter speed, and the ISO. If that approach doesn't appeal to you then all you have to do is turn the ISO dial to C where it will lock and the shutter dial to 1/3 step where that dial will lock and you have a fully electronically controlled camera with all the bells and whistles. Note if you want to go fully electronic I would suggest you look into the Z6 III, as a stills camera it's equal to the Zf and it does have less Rolling Shutter than the Zf.
Auto ISO is a useful tool, and when set up properly, functions the way it is designed to do and will produce a very good image. Should it be used ALL the time? IMHO, no. And I don't use it all the time. However, I DO use it when the need arises. BUT--here's the key to stop the problem you cite as the aperture not working the way it should--you HAVE to set it up correctly. If not, then you'll get the sort of issues that you're talking about, as well as others. But to say one shouldn't use Auto ISO at all is, IMHO, bad advice.
Third. Lets talk about Release Modes. The hidden aspect of this is Rolling Shutter which will give you a nip in the backside on each shot for subjects moving rapidly or with rapid pans. For either the Z6 III or the Zf the best way to avoid Rolling Shutter is to NOT use the electronic shutter for fast moving subjects. On the Zf this will limit you to 14 fps, which I really don't consider a burden. Usain Bolts 100 meter sprint record worked out to about 23 mph for average speed. You are planning to taking pictures of School Age participants so do you really have a need for more than 10 or 15 fps? Not really.
Rolling shutter can certainly be a problem, and while I don't have a Zf or a Z6 III and haven't used one of them, I can't say just how well or not they handle that particular issue. But, once again, there are times when using the electronic shutter is useful and helpful (and possibly even necessary!), and times when you shouldn't do so. Knowing the camera, the situation you're shooting, and whether or not it would be appropriate for it, is up to the photographer to decide. Proper use of all of the tools available to one, instead of merely saying, "Don't use X", is a sign of growth as a photographer.
Four. Low Light shooting. Currently the Zf is the finest Low Light camera that Nikon makes. BTW the second best is the Z6 III and that is only because of that 1/2 stop of dynamic range advantage the Zf features. If you are shooting above ISO 800 they are complete equals. Both camera will focus down to -10 EV, which is less than the light of 1/4 moon.
Summation. I would suggest that you save a considerable amount of money and choose either the Zf or the Z6 III. The major difference between the two is the 500 dollar price differential and the Z6 III's video capabilities. A sort of minor difference is the Z6 III has less Rolling shutter than the Zf. If you stick to the mechanical shutter Rolling Shutter is out of consideration. Yeah, the Zf has a tiny advantage in Dynamic Range but it's so slight I don't think it's worth considering.
IMHO, the whole control layout and design of the Zf may or may not be a MAJOR factor in choosing one over any other camera. Some people like the old-style SLR control layout and form factor and others don't. But that's a pretty major factor when deciding a camera. Modern DSLRs and mirrorless bodies (at least from a certain price point on up) are designed to be able to change MANY controls and settings without ever taking your eye away from the viewfinder. That's not always the case with the Zf, as I understand it.
Basically it comes down to price and what you prefer in a Still's camera. Me, I chose the Zf because I find it so convenient to have all of the major controls on a labeled dial. You may chose the Z6 III because you don't like that approach for controls and because you can foresee the need for short video clips. If so the Z6 III is a much better choice.
As for the Z8 as a I stated in the opening the Z8 is a fantastic camera and a good way to burn up funds that might be better spent on lenses.
PS; I also have the Z7 II and have first hand experience with 46mp and 24mp resolution files. The difference between the two resolutions is a LOT LESS critical than the numbers suggest. The plain truth is that you'll have to print with very large format printers to actually observe the difference. The only real world advantage of the higher resolution images is that they do provide for more "crop headroom" than the 24mp images. If you want to crop to 1/4 frame the 46mp will hold up better to the increase in magnification. However a good photographer will only have to resort to that deep of a crop if they failed to bring the correct lens for the subject.
YMMV, of course.
Sam