If it's coming out dry and cooked, you're smoking it too long. At 175-180* I would think 4.5 hrs wouldn't be too long, but it sounds like it. When you let it air for 1.5 hrs before smoking it, that also starts drying the meat. Once you've pulled it from curing, throw it into the smoker without airing it. Proceed with the moisture dumping every 45 minutes or so, and watch it. It could be ready anytime after 3.5-4 hrs., but most likely will go longer.
Mine goes for 4.5-5 hrs, but I use a wet cure typically.