Some Advice About iPhone
I think the back of the iPhone could definitely be made flat. The added thickness could allow for a larger battery, and some of the internal layout logic could be reworked. Overall, it would be better to keep a flat back; being a bit thicker is perfectly fine. There’s no need to chase an ultra-thin feel, especially when the rear camera protrudes from the body anyway.
iPhone 12 Pro Max, iOS 26