Cu₃SbSe₄ compounds with a diamond-like structure are promising eco-friendly p-type thermoelectric materials. However, their thermoelectric performance is limited by low intrinsic carrier concentration and mobility. This study employed GeMnTe₂ doping to simultaneously regulate the electronic band structure and carrier transport properties of Cu₃SbSe₄. First-principles calculations show that Mn ions introduce a resonant state (3d orbital) and promote band convergence, enhancing the density-of-states effective mass near the Fermi level. Concurrently, GeMnTe₂ doping induced the lattice relaxation effect and promoted the transformation of the matrix structure to a pseudocubic structure with higher symmetry, which reduced the deformation potential and inhibited the decreasing in carrier mobility under heavy doping. Multi-scale defects, including point defects and Ge/MnO_x nanoprecipitates, strongly suppressed the lattice thermal conductivity. Therefore, the power factor of 11.5 μW·cm⁻¹·K⁻² was reached by the 1.5% (in mole) GeMnTe₂ doped sample, and the peak ZT of ~1.2 was obtained at 723 K. Meanwhile, the hardness and modulus of the optimized sample were increased by 10%. These findings indicate that the electrical transport properties of Cu₃SbSe₄ can be synergistically optimized by band engineering and lattice symmetry tuning. This study provides a valuable method for the design of high-performance diamond-like thermoelectric materials.