In this study, precise control over the thickness and termination of Ti₃C₂T_X MXene flakes is achieved to enhance their electrical properties, environmental stability, and gas-sensing performance. Utilizing a hybrid method involving high-pressure processing, stirring, and immiscible solutions, sub-100 nm MXene flake thickness is achieved within the MXene film on the Si-wafer. Functionalization control is achieved by defunctionalizing MXene at 650 °C under vacuum and H₂ gas in a CVD furnace, followed by refunctionalization with iodine and bromine vaporization from a bubbler attached to the CVD. Notably, the introduction of iodine, which has a larger atomic size, lower electronegativity, reduce shielding effect, and lower hydrophilicity (contact angle: 99°), profoundly affecting MXene. It improves the surface area (36.2 cm² g⁻¹), oxidation stability in aqueous/ambient environments (21 days/80 days), and film conductivity (749 S m⁻¹). Additionally, it significantly enhances the gas-sensing performance, including the sensitivity (0.1119 Ω ppm⁻¹), response (0.2% and 23% to 50 ppb and 200 ppm NO₂), and response/recovery times (90/100 s). The reduced shielding effect of the –I-terminals and the metallic characteristics of MXene enhance the selectivity of I-MXene toward NO₂. This approach paves the way for the development of stable and high-performance gas-sensing two-dimensional materials with promising prospects for future studies.