Adhesive joints are a crucial technology for efficiently bonding dissimilar materials, offering a superior strength-to-weight ratio compared to conventional fastening methods. Despite their widespread industrial use, a comprehensive understanding required to unlock their full potential remains lacking. This gap is primarily due to the complex influence of adherends on the adhesive's curing process, which alters its nanoscale microstructure and mechanical properties. To address these challenges, we introduce a reactive coarse-grained molecular dynamics (MD) model to investigate the formation, load-bearing mechanism, and failure of epoxy-based adhesive joints. To this end, we calibrate coarse-grained potentials for alumina adherends and combine them with a reactive epoxy model from the literature, serving as the adhesive. We derive the missing interactions between the adherend and adhesive by matching the coarse-grained and atomistic free energies obtained via metadynamics. This ensures that the entire adhesive joint model is grounded in all-atom simulations, following a bottom-up approach. We investigate how chemical bonds between adherends and adhesives (grafting), as well as the mixing ratio of resin and hardener in the adhesive, significantly influence interphase formation, and ultimately affect the stiffness, strength, and toughness of the joint. Notably, we find that slightly resin-poor mixtures effectively mitigate interphase formation, optimizing both strength and toughness. This study presents an advanced simulation framework to elucidate the complex structure-property relations of adhesive joints, facilitating the development of next-generation adhesives with improved performance.