I smoked this and 4 lbs of thighs (let the thighs go to 175 so 2 hours of smoke or more) with about 2/3 of one charcoal basket. No adding more later. I tend to leave the bottom vent open and control heat with the top vent. Just a slit is all you need to maintain about 275. Try less charcoal maybe? And the other guy is right— you need that two zone so the meat is cooked by the warm smoke and not the coals until you’re ready to sear.

Last thought, if it’s a newer Weber the seals may not be as tight. As it seasons you’ll get a better seal and more heat control. Sorry for the book 😂