I have done some experimenting with my WSM 14". The dome therm, like most stock therms is off. So don't use that for figuring out your pit temp. For chicken I'd remove the water pan completely from the smoker. You should have no problems achieving temps above 400°. Good therms at grate level are in my opinion a must. With that said when I tested the dome and grate with my therm probes the dome was within 8° of the grate (stock therm was off by 25°-30°).
What method are you using to light the smoker? I have found that the best method for the WSM family is the use what is called the side light method. Using this method you fill the charcoal basket completely full. Place your smoke wood on top, Assemble the smoker, leave all vents wide open. light a propane torch and insert it into one of the lower vents. run the torch until you are 25° above your target temp. remove the torch. The temp is going to drop considerably. once it stabilizes repeat the torch. If you are going for a low temp smoke (under 285°) torch in the same vent. If going for a high temp smoke use one of the other vents. This time only go over by 10°-15°. Temp will drop. Allow pit to come up to your target temp. Adjust lower vents as needed. Leave dome vent wide open always. If you need help with that let me know.
Now for seasoning. On your next bird try this simple rub of Salt, Pepper, onion powder, garlic powder (AKA SPOG) then expand on it every cook. You don't need a bunch. The rub in your first photo looks pretty heavy. Keep a log of what you used. Modify and make adjustments. As for brining go for it if you arew looking to change teh flavor profile. Just remember its not needed for a moist bird. Also if you brine, try it with just the brine then add a rub. You may find that your rub and brine don't compliment each other.
Cherry, apple, kiawe, pecan, alder, peach are all good woods to use on poultry. Smoke should be really light and bluish to no color.
One last tip (you've already got the spatching down) Air Dry the bird. Best to do overnight uncovered in the fridge. If you are short on time, add 1/4 teaspoon of baking powder to your rub and apply the rub 2-4 hours in advance the baking powder and salt (SPOG) in the rub will help to dry the skin out. Dry skin will help get it crispy, or at least bite through. Okay, one more, don't over cook the bird! 165° in the thigh or in the breast will get you a moist bird.