To begin with, just so you are aware, if you are using Disney transportation to the port, you will not be able to select a PAT. PAT is only something you can choose if you are driving or taking transportation arranged through a third-party.
You will be able to check in online and select a PAT sometime in the late afternoon/evening before your scheduled onboard activities booking date (i.e. 75 days before sailing). So if your onboard activities booking date that shows up in the
Disney cruise website/app is September 10, the check in would be available sometime in the evening on September 9 (onboard activities will then be come available to book later on that night at midnight eastern).
I think there is a good chance of getting a fairly early PAT as a first timer. I am on a cruise where we are between the silver and first-timer check in dates, and there are still plenty of times available for first timers. They don't seem to book up very fast, likely because so many people are on
DCL transporation, and also because some cruisers like to arrive later so they don't have to wait in the terminal and are closer to when the room is ready.