I would maily say gentics comes first for hard nugs before any thing else. Then the many variables in the grow room would come next, good air flow is no1 for harder nugs, light penetration through the canopy and of course light intestacy. Spectrum will play big part as well.
Looking at your set up your definitely on the ball. So in my opinion gentics is your main worry. I wouldn't be buying some bud hardener.