As PP mentioned you can have a Port Arrival Time assigned after/during online check in, which I thought everyone, including first timers could already perform. So much for the time passing quickly till Oct 20th.

As PP said once paid in full and at 75 days out, you can book ressies at 12:01 eastern time, then check-in is at 3:01 where you get PAT time. This PAT time slot is related to your actual boarding # assigned at port IF Not using
DCL transport.
So if I understand, you will be checking in a few days after 75 days out. So the other 3994 guests could/may obtain a PAT before your party.

We also play the CC cycles.
It's OK, not everyone knows to do online check in ASAP. Also even if you show prior to your PAT they won't turn you away unless the terminal is at capacity. If assigned a late PAT time it only means you will have to wait till general boarding is called, which IMO would be around 12:45 - 1:00. Also many try to plan to arrive after open boarding to avoid a crowed terminal. It can get crowed between 11 - 1. The trick for most is to arrive after open boarding and prior to the DCL buses from the resorts, which could start to arrive at 1:30.
Online check-in at 3am has 5 or 6 steps of info required prior to PAT assignment. These included; Guest info(names), ID(passport or DL), Pre / post details(when arriving, where staying, flight #, airline, etc.), Onboard acct. info(using CC or cash), and Accept T&C(check box).
Most skip what they can and edit later. I said cash for OBC, Driver License, etc. etc. just so the page will ungray and open the PAT section.
The reservation section can be done at 12midnight but has a weird quirk. After you select an item for your cart you must click "confirm". Also if you look back at availability the item still shows available to you, but only you. It makes people think "it" didn't take or something. Also DCL site prefers Firefox browser fwiw.
Wow that was long
HTH and Good Luck