In situ phase separation precipitates play an important role in enhancing the thermoelectric properties of copper sulfides by suppressing phonon transmission. In this study, Cu_1.8S composites were fabricated by melting reactions and spark plasma sintering. The complex structures, namely, micron-PbS, Sb₂S₃, nano-FeS, and multiscale pores, originate from the introduction of FePb₄Sb₆S₁₄ into the Cu_1.8S matrix. Using effective element (Fe) doping and multiscale precipitates, the Cu_1.8S+0.5 wt% FePb₄Sb₆S₁₄ bulk composite reached a high dimensionless figure of merit (ZT) value of 1.1 at 773 K. Furthermore, the modulus obtained for this sample was approximately 40.27 GPa, which was higher than that of the pristine sample. This study provides a novel strategy for realizing heterovalent doping while forming various precipitates via in situ phase separation by natural minerals, which has been proven to be effective in improving the thermoelectric and mechanical performance of copper sulfides and is worth promoting in other thermoelectric systems.