In this study, we report a systematic investigation of the dielectric properties and structural evolution of K(Ca₁₋ₓSrₓ)₂Nb₃O₁₀ and H(Ca₁₋ₓSrₓ)₂Nb₃O₁₀ layered perovskite ceramics (x = 0, 0.25, 0.5, 0.75, 1) to evaluate their potential for high-temperature multilayer ceramic capacitor (MLCC) applications. While BaTiO₃-based MLCCs suffer from dielectric instability above 120 °C, the proton-exchanged series H(Ca₁₋ₓSrₓ)₂Nb₃O₁₀ demonstrated stable temperature coefficient of capacitance (TCC) performance up to 200 °C, particularly for samples sintered at 1300 °C. Among them, HSr₂Nb₃O₁₀ (x = 1) exhibited the highest dielectric constant (>700 at 25 °C, 1 kHz) and exceptional TCC stability within ±15% across the full temperature range. Structural analysis revealed that sintering induces phase decomposition into stable binary niobates such as SrNb₂O₆ and Sr₂Nb₂O₇. Interestingly, the dielectric constants of the sintered HSr₂Nb₃O₁₀ ceramics exceeded the weighted average of their constituent phases. Transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analyses revealed that strain fields and high-angle grain boundaries introduced during phase decomposition are responsible for this enhancement. In particular, the localized tensile stress at grain boundaries in HSr₂Nb₃O₁₀ (x = 1) is suggested to promote high polarization regions, leading to improved dielectric performance. These findings highlight the role of strain engineering and multiphase interfaces in designing advanced dielectric ceramics for next-generation MLCCs.