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In this work, we present a comparative first principles study of the cubic K2IrX6 (X = Cl, Br, I) halides, focusing on their structural, mechanical, electronic, magnetic, thermodynamic, and thermoelectric transport properties. Structural optimization confirms mechanical stability for all compounds, and reveals a systematic increase in lattice constant and unit cell volume from Cl to I, consistent with the larger halogen ionic radii. Spin polarized electronic calculations show strong spin asymmetry near the Fermi level, with a metallic majority spin channel and a minority spin gap that narrows along the series, reflecting enhanced Ir-X hybridization. The total and projected density of states indicate that Ir 5d and halogen-p states dominate the electronic activity around EF, while potassium contributes negligibly. Thermodynamic properties evaluated using the quasi-harmonic Debye model exhibit physically consistent temperature and pressure dependent trends for Gibbs free energy, entropy, thermal expansion, volume, and Debye temperature. Boltzmann transport calculations predict pronounced temperature dependence of the Seebeck coefficient and electrical conductivity, and the resulting thermoelectric figure of merit increases across the series, reaching its highest value for K2IrI6 due to the combined balance between the power factor and reduced lattice thermal conductivity. Overall, the K2IrI6 family emerges as a mechanically stable and electronically tunable platform with promising multifunctional behavior relevant to electronic and energy related applications.