300F works fine. If your pellet smoker can hold 325F, that's what I'd shoot for. Don't bother spritzing unless you want rubbery skin.
You'll get plenty of flavor in the meat with your brine and injection. I don't stuff my turkeys with anything, just salt and rub the cavity really well if left whole. Spatching the bird is a great idea. Smoke the backbone along with the neck. Makes for a fantastic gravy, which brings me to the rub.
For brined and injected birds, the rub is really cosmetic, more for the folks who like eating the skin, but it does flavor the drippings. I use a dried herb poultry rub called Litehouse. I have smoked all natural birds that were rubbed, but not brined or injected. The meat had a fine turkey taste, but was missing something, tasting rather bland. The gravy, made from the drippings, neck (and backbone if spatched) was fantastic though. A lot of the rub will flavor the drippings as the fat renders out of the skin. You don't have to wait until the turkey is done to start the gravy. Smoke the neck and backbone for a couple hours, then throw them in some water to make the turkey stock for the gravy.
Since I mentioned drippings, be sure to put a pan under the bird to catch them, then use the fat when you make your gravy. If I have a bird IN the pan, I put a cooling grate in or across the pan to raise the bird out of the drippings.