The Anjilsi landfill in Babol County, located in the humid region of northern Iran, generates a considerable volume of leachate daily due to high rainfall and the high permeability of landfill layers. Managing this leachate poses serious technical and environmental challenges. This study aims to present an integrated framework for analyzing, strategizing, and technically designing an optimized leachate treatment system for this site. In the first step, the actual volume of generated leachate was simulated and estimated using the HELP (Hydrologic Evaluation of Landfill Performance) model, revealing that the current treatment facility lacks sufficient capacity to handle the inflow load. Subsequently, by applying multi-criteria decision-making tools, namely SWOT, QSPM, and SPACE, the existing system was evaluated in terms of technical and managerial performance, and the strategy of “capacity enhancement while maintaining the existing structure and upgrading critical units” was identified as the optimal alternative. Based on this strategy, a modified treatment process was designed, consisting of anaerobic (ABR), chemical coagulation–sedimentation, membrane bioreactor (MBR), reverse osmosis (RO), and final disinfection (UV/chlorination) units. Mass balance analysis demonstrated that the proposed system can remove COD, BOD, ammonia, TSS, and heavy metals with efficiencies exceeding 80%, producing an effluent fully compliant with national discharge and agricultural reuse standards. The novelty of this research lies in the integration of hydrological modeling, strategic analysis, and a locally adapted process design, which can serve as a practical model for upgrading leachate treatment systems in humid-region landfills across Iran.