Conventional alloying strategies often require high alloying concentrations, leading to impurity phases and additional phase transition that limit the figure of merit of thermoelectric materials. Here, we introduce metavalent alloying and vacancy engineering as transformative strategies to facilitate the orthorhombic-to-cubic phase transition, in which we stabilize pure cubic GeSe under ambient conditions with just 10% alloying concentration using SbTe as an effective alloying agent. Compared to the covalently bonded orthorhombic phase, the metavalently bonded cubic GeSe features lower cation vacancy formation energy, reduced bandgap, enhanced band degeneracy, weaker chemical bonding, stronger lattice anharmonicity, and multiple phonon scattering centers. These properties synergistically improve the power factor and suppress the lattice thermal conductivity. Subsequent trace Pb doping further reduces the lattice thermal conductivity, achieving an unprecedented ZT of 1.38 at 723 K in cubic (GePbSe)(SbTe), along with a remarkable energy conversion efficiency of 6.13% under a 430 K temperature difference. These results advance the practical application of GeSe-based alloys for medium-temperature power generation and provide critical insights into the orthorhombic-to-cubic phase transition mechanism in chalcogenides.