Among the many potential Eu²⁺-activated sodium superionic conductor (NASICON)-based host materials, the Sc³⁺-based NASICON phosphor (Na₃Sc₂(PO₄)₃:Eu²⁺) is a promising phosphor material for high-power lighting applications owing to its unusual thermal stability at elevated temperatures. It has previously been shown that negative thermal quenching (TQ) can be tailored to zero TQ depending on the Eu²⁺ concentration. However, the obtained zero-TQ composition has low photoluminescent quantum yields, which hinders its applicability to high-power lighting. Herein, we report a holistic study of the tuning of thermal stability from negative TQ to zero-TQ while preserving the original emission efficiency by introducing Lu³⁺ ions in Na₃Sc₂(PO₄)₃:Eu²⁺. Furthermore, we fabricated a high-power white light emitting diode (WLED) using optimized Lu³⁺-doped Na₃Sc₂(PO₄)₃:Eu²⁺ as the blue component, delivering a high color rendering index value of 90 with a high luminous efficacy value of 25 lm/W obtained at a flux current of 1000 mA. Therefore, the findings of this work provide novel scientific insight into the importance of structure-property relationships in designing highly efficient thermally stable phosphors for high-power lighting applications.