I'm an annual passholder who just moved here from Iowa a year ago. This information is important because I have the perspective of an occasional long distance traveler and have the respect for the sacrifice it takes to get down here, and the perspective of a local who can go frequently on a last minute whim.
We used to bring our large family to WDW for an every other year vacation. We have 9 in our family, so we never stayed onsite. We have this year with our smaller family though. So I have the perspective of staying on and offsite too.
You might consider staying offsite now that your family is getting larger. We have rented houses through VRBO, stayed at the Blue Heron, and at the Mystic Dunes in Celebration. We will be staying there, Mystic Dunes, for a family get together with our kids and their spouses and grandkids this summer, all totalling 14 now.
The reason we stayed offsite is we could get our rooms for under $150 a piece and sleep all of us, with the exception of the Mystic Dunes which came to us as we inherited my mother-in-law's timeshare (which I wish we hadn't due to the fees and taxes).
We always have/had kitchens/kitchenettes for making meals, and we eat off site at Sweet Tomatoes. The kids can eat until they are full at for a fraction of the costs, and get healthier food. That said, however, I know a lot of people that go to Disney for the food experience, but I always saw it as a waste of time and money. Everyone is different and I respect that.
I know, also, that some of the savings of the cost of a rental car by being offsite takes away some of the savings of staying offsite, but there are sooooo many great places to visit in Central Florida on your off days and having a rental car makes that possible. A day or two break from the parks is rejuvenating. See my non-monetized blog wdwlens.blogspot.com for ideas.
Now, about the crowds.... I go probably every three weeks with my three remaining kids that haven't left the nest. My experience this year is that there has not been one day that has thrilled me due to lack of crowds. Someone has seemed to gotten the memo that such and such a time is not crowded and now they are coming when it's "not crowded" so now it's crowded. Plus, touring plans lately needs another couple of years with the new pricing, and other changes to be able to be accurate. I have found their crowd levels to be quite consistently predicted to be lower than what was expected. Take this last Monday, for instance, MK was predicted to be a five and it was......an....8!!!!!!! It was horrible.
Wednesday MK was predicted to be a 4 or 5, I can't exactly remember, but it was a 7.
On October 13th AK was predicted to be a 6 and it was a 9. And January 6 it was predicted to be a 6 and it was a 9. I have not had any luck on my days since August.
However, I have learned that a crowded day at WDW is better than no day at WDW, and one needs to lower expectations, bring your patience, and get your Fast Passes for the middle of the day when it will be hot and you don't want to be standing in long lines. It can get hot in December too.
Get to the parks at rope drop and ride like mad people your favorite rides until 10:30. At 11:00 go off site and get lunch. Come back and ride your FP rides from 1 or 2 to 4 or 5, and then get a snack in the parks, or dinner, and then ride rides during the parades, and stay late....if you have the energy. On a 9 level day we rode Everest single rider 4 times from 7:00 until 8:00.
Pay more attention to the hourly crowd levels and plan around those as opposed to daily crowd levels because it is possible to have fun on a level 9 day if you do that. Don't plan your several thousand dollar vacation on a prediction that may or may not happen.
Now, that being said, yes, you and I are both used to the South, but beings that we can't depend solely on the crowd predictions, I would suggest the end of November and the beginning of December, or the end of January and the beginning of February, just because a cooler day is going to make an unexpected crowd more tolerable. Heat and crowds = misery. Don't go during the Princess Half Marathon, or anytime after the third week of December, or the third week of March to the third week of April. And summer is out until the third week of August.
I hope I don't sound too negative. I just don't want you to have your bubble burst.
I have just heard so many people as I walk by murmuring because they didn't expect the crowds. They planned to and expected to walk on. I'm afraid those days are over. I don't mean to be a pessimist. The only way I could know for sure is to attend every day, but so far, it's been consistently busy on the the days that were predicted to be low, so I've learned to use the hourly predictions to deal with it.
Also, be willing go to the old attractions to fill in, i.e. People Mover, Carousel of Progress, Country Bear. You will fill more accomplished and more satisfied the more attractions you get in, and if you aren't standing in line for 2 hours for the Mine Train, you will feel better about what you have accomplished.
Those old attractions are still great and nostalgic.