Try to clean connector and reseat the screen flex connector maybe it don’t seat propperly. If that doesn’t help then you have damage the screen flex cable.
After 2 years the moister is vaporised but corrosion on the board must be very bad. I think that the chance is very very small that it can be fix, because the corrosion has damage the logic board to much in those years. Good luck if you continue the fix.
Your tabs are stuck in the phone, you have only lift up setje front of the screen and the metal frame of the screen is still inside the iPhone. You first got to get them out before you can instal your new screen.
It is hard to say if you don’t open the phone, so the best you can do is open up the phone and look if the logic board isn’t broke. If logic board isn’t broke then I would try a new screen, these aren’t so expensive anymore.
You wrote: the iphone only seems to turn on when I connect it to the computer, as soon as I pull the usb cable out the iphone also turns off. While the battery was in good condition. So i think the best you can do is try a new battery because the other things you allready tried.
adhesive metal spacers