We haven't been in early December in awhile, but the later you go in December, the more crowded it will become.
It is going to be very crowded, much worse than it is now. Summer is actually off peak at Disney. During our trip with similar dates last year, FOP had somewhere between at 3-5 hour wait depending on the day. Early hours are nice, but it is true the park seems more crowded as the day goes on. We typically don't take advantage of early hours.