The usual. Faster, better camera, the various new features like the satellite and emergency communications. The high refresh rate display might be a really good improvement.

The small upgrades build over the years, so you would notice a difference. But it’s nothing massive like when they went from LCD to OLED or added the plus size.

There’s nothing wrong with an 11 Pro. If it’s still working for you it’s perfectly reasonable.

Not that I can really see. I’m still using an iPhone X and don’t really feel a need to upgrade yet.