You have the two cameras I have as well. Here’s my initial take on things.
1) Most sports and events that I shoot have plenty of dead time between innings, quarters, halves, timeouts, sets, songs, heats, races, dances, riders, you name it, to allow me to swap out batteries and not miss anything. However, I don’t like swapping batteries out in wet weather, and I still find it easy to shoot a high number of frames at 20 FPS. So I wouldn’t give up the Z9 and its bigger battery as my main body.
2) From what I’ve seen, the Z8 doesn’t have the voice memo button, but it lists the option in its specs.
3) I think the human brain is conditioned to enjoy seeing people framed in portrait orientation. I can get there through cropping, but I would miss the function of the Z9 grip, especially in higher ISO territory where cropping brings about more noticeable higher noise. And from what I’ve seen of the Z8 add on grip, I wouldn’t want to mess with that thing.
4) I would certainly welcome holding less weight for hours at a time, or having less hang from me. But I wouldn’t prioritize that pound for a main body. A second one, yes.
I look at the Z8 as a potential replacement for what the Z 6II is for me, not as a replacement for the Z9. But before I upgrade the Z 6II, I want to see what comes of the Z 6III. I would love for that to be a lower megapixel Z8, but I doubt it gets the stacked sensor.