I don't think you need an action camera like the Z8. I do think the other aspects of subject recognition and improved AF would be very useful. For portraits, this camera seems very good - and it's ideal for casual use, travel, family events, etc. Compared to the Z5, you are getting faster frame rate, significant updates to AF, better video capabilities, the flip screen changes, pre-release capture, and retro styling/controls. For any kind of wildlife subject, the only shortcoming is fast action like birds in flight - and even then it is for unpredictable action. Fast action that is moderately predictable will be fine and with subject detection and tracking improvements, will likely be better than your Z5.I own a Z5 and mainly shoot macro and portraits I'm thrilled that this has a screen (flip out or 3 way tilt I am not bothered) that I can use for low level portrait orientation photography that I don't have to pay 4000 bucks for (Z8). I only shoot garden birds once a month or so which means I think this could be the camera for me. As a non hardcore bird photographer I think the Z8 would be too much camera for me. The question is would the Zf be a step up from the Z5 for the occasions I shoot small birds (not in flight). It am sure it will be a step up for everything else I use it for.
Of course I could wait for a Z6iii but who knows when that will come and what the price will be.