The development of efficient and stable solid electrolytes remains one of the central challenges in realizing high-performance sodium-based solid-state batteries. Among the emerging material classes, mixed-anion compounds have attracted increasing attention, as the deliberate incorporation of multiple anionic species enables fine-tuning of both the structural framework and ion transport properties. In this contribution, we investigate a family of sodium metal oxyhalides derived from the NaTaOxCl6-2x series to explore how systematic anion substitution affects the local structure, bonding environment, and Na+ transport behavior. By combining advanced mechanochemical synthesis routes with X-ray diffraction, solid-state NMR, and spectroscopic analyses, complemented by atomistic simulations, we establish fundamental correlations between the locally mixed transition-metal coordination, framework flexibility, and ionic mobility. The study provides a comprehensive perspective on how variation of the oxide-to-halide ratio reshapes the energy landscape for Na+ migration and governs mechanical as well as electrochemical stability. Demonstrating solid electrolytes such as NaTaO0.5Cl5.0, which exhibits a room-temperature ionic conductivity of approximately 4 mS·cm⁻¹ and improved high-rate performance, highlights the promise of this material class. The presented findings open new pathways toward the design of flexible oxyhalide solid electrolytes that can serve as catholytes in advanced sodium solid-state batteries.