Interest in high-permittivity dielectric materials suitable for classical systems has been increasing, and competition for commercial applications continues. However, despite the development of such dielectric materials, additional compositional improvements are still required to achieve low-temperature sintering that would allow co-firing with Cu internal electrodes in multilayer structures, and research in this area remains insufficient. In this study, we aimed to optimize a low-temperature sintering composition based on Pb0.94La0.06(Zr0.83Ti0.17)O3, which, in preliminary experiments, exhibited a dielectric constant above 1,300 at sintering temperatures exceeding 1300 °C. As the amount of Na ion addition increased, low-temperature sintering was effectively promoted. However, the dielectric constant simultaneously decreased. When K ions were further added to the Lead Lanthanum Zirconate Titanate (PLZT)+Na composition, the low-temperature sintering properties were maintained or improved, and the dielectric constant increased compared with Na-only addition. To precisely readjust the MPB region under the influence of large additive content, the Zr ion fraction was varied from 0.84 to 0.92. Within this range, the MPB composition was found to shift slightly as the Zr content increased. As a result, the optimal composition among the PLZT ceramics sintered at 950 °C was determined to be Zr 0.86, which exhibited a dielectric constant of approximately 900 and an energy storage density of about 2 J/cm3. These findings suggest that such a composition could be applicable for low-temperature co-firing with Cu electrodes in Multi-Layer Ceramic Capacitors (MLCCs) for classical systems.