NASICON-type Li+-ion-conducting solid electrolytes have emerged as promising materials for enabling safer and more stable solid-state lithium metal batteries (SSLMBs), thereby effectively addressing key challenges associated with conventional batteries that use flammable and thermally unstable liquid electrolytes. Among them, LAGP offers a favorable balance of structural stability, high ionic conductivity, and a wide electrochemical stability window, validating its viability as a promising solid electrolyte for SSLMBs. In this study, LAGP solid electrolyte was synthesized through a solid-state reaction at significantly lower temperatures (∼800 °C) than those employed in melt-quenching approaches (>1300 °C). The effects of calcination and sintering temperatures on the phase formation, hardness, density, and ionic conductivity of LAGP were investigated. LAGP sintered at 775 °C exhibited the highest ionic conductivity of 2.6 × 10−4 S cm−1, attributed to improved phase purity and densification. LSV measurements revealed a wide electrochemical stability window of 5.8 V vs. Li/Li+. A liquid-assisted solid-state lithium metal cell with an LFP/LAGP/Li configuration was assembled, delivering a discharge-specific capacity of 155 mA h g−1 at 0.1 C. Finally, XRD, XPS, and impedance measurements were performed after the rate studies to evaluate the stability of LAGP at the electrode interfaces.