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Substitutional doping is increasingly recognized as the most viable and stable approach to tune carrier concentration or impart added functionalities in 2D transition metal dichalcogenides (TMDs) and represents the next step towards realizing 2D TMD-based field effect sensors, transistors, and quantum photonic devices. In this work, we report on the role of substitutional metal doping, emphasizing Re (Rhenium) dopants, in modulating the structural and (opto)electronic properties of 2D MoS₂. Using metal-organic chemical vapor deposition (MOCVD), we achieved uniform doping in MoS₂ films with controlled Re concentrations. Initial results demonstrate that dilute Re doping effectively reduce sulfur-site defects in MoS₂, thereby improving electronic transport properties. However, excessive Re doping led to reduced n-type doping, increased strain in the film, and broad emission due to Re-related defects, attributed to dopant clustering. Ab initio calculations reveal that when Re dopant atoms aggregate, their ionization energy rises, leading to a decrease in the doping efficacy. This clustering effect is confirmed by scanning tunneling microscopy (STM) measurement showing that the transition from isolated Re atoms to Re clusters leads to a higher percentage of unionized dopants in the neutral charge state. Photoluminescence (PL) measurements demonstrate that Re clustering also introduces new defect states that trap photogenerated excitons, and give rise to broad sub-gap emission. While Re doping provides a potential avenue to enhance MoS₂'s (opto)electronic properties, it's essential to maintain a careful balance to avoid the negative effects of excessive doping and clustering. The insights gleaned from our research furnish a deeper understanding of how local metal dopant concentrations modulate carrier density, defect generation, and exciton dynamics in TMDs.