Betty, I will be honest with you, I am not well-versed in doing high-quality scans with a macro lens. So take what I say with a big grain of salt.
If you want the best quality scans with minimal issues and good edge-to-edge sharpness, I would recommend to get a modern macro lens and shoot at apertures faster than f/5.6 to get the best sharpness. Older lenses can be good, but they are not made for digital sensors, especially high-resolution sensors like on the Z7. When I tested a few macro lenses, I found that older lenses have some serious problems with CA at close distances, and they generally have issues with sharpness outside the center area, especially when shooting at macro distances. For the Z-mount, you might want to wait until the 105mm f/2.8 macro is released - it is supposed to be pretty incredible, and made for a 45MP+ sensor. But if you have a deadline, then give the Nikon 60mm f/2.8 a try, and let me know how it works out.
As for bellows, other than being able to mount the slide copy adapter, why would you use them? Is that the only way you can attach the slide adapter? Didn't Nikon make ES-2 adapter that mounts directly on the D850? I think that would simplify things?