[JJ_SO]

On Z 50 you push the ISO button and turn on or off Auto ISO with the front dial or select (and override at the same time, if AutoISO is active) the ISO value with the rear dial. I fail to see a big difference? Especially the override bit is new to me, I can't do that on Z 7/6

Thanks for this find... I honestly simply missed this feature in the manual. So, can we agree that the Auto-ISO with the Z50 is not as bad as I thought, while the implementation in the X-T30 it's more flexible than in your X-T2?

Sure. Not knowing the X-T30's behaviour and not want to read another manual to find out, I can only add "why does Fuji need to change their menu systems all the time?"

On rare occasions I DON'T use AutoISO, such as studio works, architecture and landscape - everywhere else AutoISO is active as I do not care much about noise but do care about aperture and shutter speed. Noise is hardly an element of composition, at least in most pictures - the other parameters are.

Well, of course shutter speed and aperture are the main tools, same here. I do however prefer to stay as low as possible with ISO and then set aperture (usually) or shutter speed (only when the subject requires) accordingly.

Given the camera also doesn't run to max out ISO as there's only one ISO value possible to a given aperture / shutter speed combination, I can't see an advantage to set this very ISO value manually. 
Except maybe (and that again is X-T2 / X-E2 behaviour) that the lower ISO (Fuji calls that here "standard sensitivity") are also limited by my settings, like AutoISO 1: 200-800, Auto ISO 3: 1600-12800. So, if I'm in AutoISO 3 I fist need to switch to a lower AutoISO or manually choose a lower setting - that's quickly done with X-T2's dial and much less quickly with X-E2's menu. The ISO dial however... 
I could show you two pictures, one taken with ISO 200 and the other with ISO 280 or one with ISO 6400 and the other with ISO 8000. And I doubt you can tell the difference. This ⅓ step between ISO is just another symptom of brainless design. While A, L and H are just neighbours on the dial and easy to confuse, Fuji users apparently need to be able to adjust ISO by ⅓  EV step.