All-solid-state batteries (ASBs) are promising candidates for next-generation energy storage systems due to their enhanced safety and potential for higher energy densities. However, achieving practical ASBs with energy densities surpassing those of state-of-the-art lithium-ion batteries (LIBs) requires the development of thin, mechanically robust solid electrolyte separators (SESs). In this study, a scalable tape casting method is employed to fabricate a thin SES with a thickness of 27 µm and a high ionic conductance of 146 mS cm−2. The SES, composed of Li6PS5Cl SE and a laser-drilled porous polyimide (PI) scaffold with a high porosity of 69%, exhibits a tensile stress of 7.15 MPa at 6% strain, demonstrating the mechanical integrity necessary for commercial roll-to-roll fabrication. Due to its reduced thickness, the LiNi0.83Co0.11Mn0.06O2||Li-In pouch cell achieves outstanding estimated cell-level gravimetric and volumetric energy densities of 322 Wh kg−1 and 571 Wh L−1, respectively, demonstrating its practical viability. Additionally, simulation studies highlight the importance of optimizing the porosity and pore distribution of porous scaffolds to minimize Li-ion flux heterogeneity and prevent non-uniform Li plating in scaffold-supported SESs. Finally, a 4 m long, double-side coated SES is successfully manufactured using an industrial-level comma coater, demonstrating the feasibility of the approach for large-scale SES production and the forthcoming commercialization of ASBs.