That sounds really manageable for weight. At least you won't be packing out your own personal pfd, dry bags and ammo can with junk - that can get pretty heavy.
Be prepared for it to possibly be horribly hot at the bottom in August, but as stated the Bright Angel Trail is not super steep overall and it will be cooler up higher. The steepest stretches on the trail are probably not much more than 500 vertical continuous anyway and there are basically three of those, so just do you hills more than once. It is a fantastic trip and talk of the hike out is overdone.