GeTe and its derivatives constituting Pb-free elements have been well known as potential thermoelectric materials for the last five decades, which offer paramount technological importance. The main constraint in the way of optimizing thermoelectric performance of GeTe is the high lattice thermal conductivity (kappa(lat)). Herein, we demonstrate low kappa(lat) (similar to 0.7 W/m.K) and a significantly high thermoelectric figure of merit (ZT = 2.1 at 630 K) in the Sb-doped pseudoternary (GeTe)(1-2x)(GeSe)(x)(GeS)(x) system by two-step strategies. The (GeTe)(1-2x)(GeSe)(x)(GeS)(x) system provides an excellent podium to investigate competition between an entropy-driven solid solution and enthalpy-driven phase separation. In the first step, small concentrations of Se and S were substituted simultaneously in the position of Te in GeTe to reduce the kappa(lat) by phonon scattering due to mass fluctuations and point defects. When the Se/S concentration increases significantly, the system deviates from a solid solution, and phase separation of the GeS1-xSex (5-20 mu m) precipitates in the GeTe1-xSex matrix occurs, which does not participate in phonon scattering.. In the second stage, kappa(lat), of the optimized sample is further reduced to 0.7 W/m.K by Sb alloying and spark plasma sintering (SPS), which introduce additional phonon scattering centers such as excess solid solution point defects and grain boundaries. The low kappa(lat) in Sb-doped (GeTe)(1-2x)(GeSe)(x)(GeS)(x) is attributed to phonon scattering by entropically driven solid solution point defects rather than conventional endotaxial nanostructuring. As a consequence, the SPS-processed Ge0.9Sb0.1Te0.9Se0.05S0.05 sample exhibits a remarkably high ZT of 2.1 at 630 K, which is reproducible and stable over temperature cycles. Moreover, Sb-doped (GeTe)(1-2x)(GeSe)(x)(GeS)(x) exhibits significantly higher Vickers microhardness (mechanical stability) compared to that of pristine GeTe.