Context <p>Alloyed core/shell quantum dots (QDs) provide a platform for composition-controlled modulation of electronic structure and optical response. Using an integrated extended tight binding (xTB) and simplified Tamm Dancoff approximation (sTDA) framework, a composition-defined GaInZnP/ZnSe_yS_1-y core/shell QD series (<i>y</i> = 0.00, 0.25, 0.50, 0.75, 1.00) revealed a distinctly nonlinear dependence of electronic and excited-state behaviour on shell composition. Relative stability favoured S-rich shells, while mixed-shell compositions showed non-ideal energetic behaviour with a pronounced deviation at <i>y</i> = 0.75. The frontier electronic structure did not vary monotonically with Se content, while the optical response showed composition-dependent redistribution of low-energy transitions. Benchmark comparison with density functional theory (DFT) and time-dependent density functional perturbation theory (TDDFPT) confirmed that the nonlinear evolution of frontier-level separation and absorption behaviour was an intrinsic feature of the alloy series, although the exact magnitude and position of the extrema remain method dependent. A distinct anomaly at <i>y</i> = 0.75 indicated enhanced frontier state reorganisation within the mixed-shell environment.</p> Method <p>Atomistic GaInZnP/ZnSe_yS_1-y QDs spanning <i>y</i> = 0.00 to 1.00 were analysed under identical structural constraints to isolate shell anion substitution effects. Ground-state electronic structure and excited-state properties were evaluated using xTB and sTDA, with DFT and TDDFPT used as independent benchmarks for frontier electronic structure and optical absorption trends. Composition-dependent changes in energetic stability, frontier-orbital separation, density-of-states like distributions, absorption onset, and frontier-orbital localisation were then compared across the full alloy series using a consistent analysis framework.</p>