The iOS operating system remains the same no matter what model you have because there are no substantial differences graphically and software-wise.
Apple should refresh the home screen by redesigning the icons that are the same since iOS 7 (12 years always the same icons) add new gestures and more freedom of customization for example to increase the brightness devise a level also horizontally and not only vertically, add the ability to customize also the color of the battery maybe see it green when it is charged, yellow when it is below a certain percentage as well as it becomes red when it drops to 20%
It should add new animations for apps maybe that when it is opened or closed in system app it animates in the icon itself. If Apple would freshen up in this respect, I am sure we would all notice a more satisfactory user experience.
As for the difference between 12 pro and 16 pro in my opinion you're a little bit wrong because you went from a 60 hz refresh rate to a 160 hz refresh rate and already you should notice that. Needless to talk about the camera and the battery. There the 12 pro would lose hands down.