Just posting in case this helps anyone.
In my original post when we finalized our final payment at 30 days before sailing, the only check-in time available was 1pm.
But as Merion_Mom suggested, I kept checking over the next two weeks, and at around 15 days before sail date, for some reason many more earlier timeslots opened up and we managed to get 11:30am. Pretty happy with that as this now lines up with our car rental return.
So definitely keep checking as the sail date gets closer!