Band engineering of semiconductors is crucial for optimizing solar energy conversion and optoelectronic device performance. This study presents a rational approach to predicting and controlling the band gaps and composition of Cu₂GeS₃ (CGS) and Cu₂Ge(S_1-xSe_x)3 (CGSSe) alloys by combining computational methods and experimental validation. We employ density functional theory (DFT) calculations and thermodynamic modeling to investigate temperature-dependent miscibility gaps and resulting band gap changes in CGSSe, enabling the theoretical prediction of optimal processing temperatures for ideal photovoltaic applications. Cu₂GeS₃ is identified as a promising semiconductor material due to its direct band structure, abundant constituent elements, and high absorption coefficient. To address its relatively wide band gap (1.5-1.6 eV), which limits light-harvesting capabilities, we explore the incorporation of selenium into the CGS structure to form CGSSe alloys for band gap tuning. Our theoretical predictions suggest an ideal band gap of 1.16 eV can be achieved at an annealing temperature of 900 K, aligning well with experimental measurements yielding 1.24 eV under similar conditions. This close agreement validates the effectiveness of our proposed approach for rational band engineering. By leveraging the miscibility gap of these structured semiconductors, our research offers a simple yet powerful method for controlling optoelectronic properties. This methodology can be applied to other semiconductor systems, providing an efficient alternative to traditional trial-and-error experimental approaches. Additionally, we address the need for alternative materials to replace scarce or toxic elements common in current high-efficiency solar cell technologies. By focusing on more abundant and environmentally friendly elements, this work contributes to developing more sustainable and scalable photovoltaic technologies.