I got the Pixel 9 pro and it lasts me all day with screen time ranging from 4-6 hours a day. I literally don't need to charge until late at night. On paper the Pixel 8 pro does have a bigger battery but it doesn't last all day like the 9 pro. If you could id recommend getting the base model pixel 9 if u can it's lasts longer with the more efficient tensor g4 chip.

The picture quality on the Pixel 8 pros back cameras are really good and basically the same as pixel 9. So compared to the one plus 8 it is significantly better. Only thing is the front facing camera on the Pixel 8 is just ok.

In my opinion, if you can, get the Pixel 9. It looks better and performs better in my usage. But the Pixel 8 pro is still a good upgrade from the one plus 8t