This article develops the Taqrib Diagnostic–Decision Model (TDDM) as a formal, multi-dimensional framework for governing Islamic intra-faith rapprochement (taqrib) in a systematic, KPI-governed manner. Building on historical initiatives such as Dār al-Taqrīb and the Amman Message, and on contemporary institutions including the World Forum for Proximity of Islamic Schools of Thought, the study responds to three deficits in current practice: conceptual fragmentation, predominantly narrative documentation, and the absence of explicit measurement and decision-support architectures. Adopting a qualitative, design-science research approach, the article synthesises literatures on conflict transformation, social cohesion, governance of religious diversity, social epistemology and digital religion, together with KPI-based frameworks in Islamic governance. The resulting TDDM is specified as a multi-layer diagnostic–decision system that encodes problem type, level, locus, layer, family, modality and time horizon, and organises them into a coherent architecture for problem classification, portfolio design and performance monitoring. Three illustrative cases local sectarian tension, discriminatory personal-status law and digital hate campaigns demonstrate how TDDM generates tailored, cross-layer intervention bundles with associated indicator families and key performance indicators. The model’s practical implications include audit and planning tools for ministries, taqrib councils and NGOs; structured design of social and legal interventions; and a taxonomy for platform governance and AI-assisted moderation. Theoretically, TDDM reframes taqrib as a system-of-systems design problem and opens a research agenda for data-informed, KPI-driven governance of intra-Islamic diversity.