Space

it seems like general water damage, the phone repair center could have been correct on the water resistance but being water proof and water resistant are too different things. I feel that when your phone got hit with the water, the water instantly started corroding the phone. If I could recommend anything clean the parts of the phone that got hit (aka most of the logic board) with IPA! If that fixes some issues then you may want to replace the camera other then that water damage is water damage.

to get into the iPad you must remove the screen

The iPad pro 1TB model has 6GB of ram, but this is to help with the extra storage, no performance gains sadly.