Great calls on the dry brining, and spatchcocking. I have no advice on American sides, but I do know about roasting.
Dry brining allows the salt to penetrate deep into the flesh. It will do so at the rate of about 1” per 24 hours, so you can dry brine a few days in advance, but it needs to be at least 24 hours before cooking. Use 1/2 teaspoon of kosher salt per pound, or 1/4 teaspoon of table salt per pound. Leave the bird uncovered in the fridge whilst dry brining, this will dry the skin and help it crisp.
Spatchcocking the bird will drastically cut cooking time as you aren’t having to heat up all the air, or stuffing in the middle of the bird. If you are concerned about the lack of stuffing, make stuffing muffins. That way everyone gets a muffin.
Put the giblets, neck, and any trimmings in the roasting pan, along with 2 onions (skin on), 2 carrots, 1 celery rib, all roughly chopped, 2 bay leaves, a few sage leaves, a few sprigs of thyme, and 2” of water (and a cup of apple juice if you want a little sweetness to your gravy). Keep the water at the 2” level by topping up with boiling water every hour, do not let it dry out, do not top up with cold water. Put the bird on the grill (or a rack) above the pan, not in the pan. This will allow the heat to circulate all around the bird.
My preferred temp for cooking poultry is 325F. Any lower, and the skin won’t crisp. Cook to internal meat temp, not time. I like to remove the bird from the heat at 155F for breast, 165F for dark meat (it will continue to cook while it rests). The different temps can be achieved by positioning the legs closer to the heat source, or by portioning the bird before cooking. As a rough guide a 14 pound bird would take about 1.5 hours at 325F, a 30 pound bird would take about 3 hours, but please use the internal meat temp.
Do a practice run. Even more than one if you are using equipment that you aren’t experienced with. Use a chicken for practice.
If you’re really worried about cooking time, start cooking early, and hold the bird at temperature in a faux cambro. That is, put the bird in your cooler after cooking (with no ice obviously). It should stay at a safe eating temperature for up to three hours in there. Leave a temperature probe in just to be safe. The skin will lose crispness, but you won’t be stressed about finishing on time. Either way, pick your poison, stress or rubbery skin.
All of my advice comes from personal experience, and BBQ guru Meathead Goldwyn.
https://amazingribs.com/tested-recipes/turkey-recipes/bbq-and-grilled-turkey-recipe
