In this comprehensive study, we investigate the properties of Cs2NaBiX6 (X = I and Br) lead-free double perovskites using density functional theory (DFT). This is the first in-depth DFT-based analysis that delves into the substantial characteristics of these materials, addressing a significant gap in the existing literature. Our analysis focuses on the optical and electronic properties of these materials. Additionally, we examine the impact of strain (tensile and compressive) on their characteristics. The results reveal that Cs2NaBiBr6 emerges as the most thermodynamically and moisture-stable variant, as corroborated by the narrower energy fluctuation range, indicating superior thermal stability. Furthermore, the DFT analysis suggests that the mechanical moduli increase as the halide changes from iodide to bromide. These findings highlight the promising potential of Cs2NaBiX6 (X = I and Br) double perovskites for applications in optoelectronic devices.