Improving the interfacial stability between layered oxide cathodes and sulfide electrolytes (SEs) and the structural stability of the cathode active material is critical for developing high-performance all-solid-state lithium batteries (ASSLBs). Coating and bulk doping are considered effective approaches; however, creating innovative buffer coatings and bulk doping using economical and scalable fabrication processes to address these issues is a comprehensive and challenging task. This article firstly proposes a coupling design of new Li₇TaO₆ (L7TaO) surface buffer coating with bulk Ta-doping for LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) by a simple one-step in situ synthesis method to tackle the interfacial issues of ASSLBs using Li₆PS₅Cl (LPSCl) electrolyte. The L7TaO buffer coating can not only promote interfacial Li-ion transport, but also effectively suppresses interfacial side reactions and reduces interfacial impedance. Simultaneously, Ta-doping into NCM811 reduces lithium–nickel mixing, thus boosting its structural stability. Therefore, the designed ASSLBs employing the NCM811 cathode coupled L7TaO-1 wt% buffer coating with bulk Ta-doping exhibit a high initial discharge capacity of 203.02 mA h g⁻¹ with initial coulombic efficiency of 85.42% at 0.1C and remarkable cycling stability up to 5650 cycles with decay rate per cycle of 0.0069% at 1C.