when you make the cranberry sauce from scratch and every vegetable known to man, it takes 7 hours
Buy the cranberry sauce and cut down on the range of veg. And get some of the others to help with prep.
A couple of years ago, DP and I cooked Christmas lunch for 19 or 20 people, at SIL's house (the only one big enough!). We arrived at about 11 and made everything bar the cranberry sauce, Xmas pud and brandy butter (bb and one of the stuffings were homemade, but done the day before). We had one middling size turkey and a couple of crowns, so they didn't need hours in the oven, and a big gammon. Everything was ready by about 3.30.
The veg were potatoes, parsnips, sprouts, carrots and cauliflower cheese. The 2 grannies present helped with peeling veg and clearing up around us, so we always had plenty of space to work. After DP had made the 2nd stuffing, a niece rolled it into balls, another niece assembled the pigs in blankets.
I was in charge of Yorkshire pud (wrong in my eyes, but they have Yorkies with any roast dinner) veg and gammon, DP in charge of turkey, gravy and bread sauce. BIL was in charge of making sure that I had G&T on tap from midday onwards. Everything was perfect and it is still spoken of as the best Christmas dinner they've ever had.
Once the bird is in the oven, there's plenty of time to do the veg imo, as long as you don't do 10 different sorts. And the gammon doesn't take any longer to boil than the turkey does to roast.
The only year I've had a problem with Christmas dinner was when the gas pressure was so low that the turkey timed to be ready at 3pm wasn't done till 6. It didn't really matter too much, by 6 everyone was so pissed and hungry they'd have eaten beans on toast.