We derive an a priori real Hessian estimate for solutions of a large family of geometric fully non-linear elliptic equations on compact Hermitian manifolds, which is independent of a lower bound for the right-hand side function. This improves on the estimates of Székelyhidi [53] and additionally applies to elliptic equations with a degenerate righthand side. As an application, we establish the optimal C 1,1 regularity of envelopes of (θ, m)-subharmonic functions on compact Hermitian manifolds.