Even though check in is now staggered you always can have the chance more than one ship will be checking in on the same night which bogs down the app and website (you can check in on either). I stayed up and check in 2 rooms in March and it took 2 hours due to app and website issues.
You will need a photo of your documents. If using passport take one pic of your photo/signature page.
A head shot that is different than your ID pic (no hats, eyeglasses are OK as long as your eyes are visible, against a blank background from the shoulders up).
travel info, flight info, how you are getting to port and back. again you can skip adding the flight info during check in to get through it quickly and then go in later to add the flight info.
credit card info (or you can just put cash option and update later)
emergency contact info
I agree about reading through the linked thread above.
MJ