In P mode, you can shift away from the camera's suggested settings while keeping the same relationship between aperture and shutter speed so (for example) if the camera decides that f/8 at 1/125th is the best setting, you can adjust it to f/4 @ 1/500th or f/16 @ 1/30th to get the aperture or shutter speed you want/need. P is my usual mode since it allows full control (especially when combined with exposure compensation) while having a fair bit of automation.
I don't get the option to set the rear wheel to control the ISO so I have the front dial (on the TZ 100 that's the ring around the lens) set for ISO and the rear for shift. Works for me!