Epoxy-based nanocomposites are widely used in high-voltage insulation systems, where dielectric reliability is governed by local electric-field distribution. In this work, gamma, and alpha alumina nanoparticles were sustainably synthesized from recycled aluminum beverage cans and incorporated into epoxy resin to investigate the synergistic effects of morphology, surface functionalization, and particle proximity. Experimental results show that breakdown strength improves with filler loading up to an optimum of 5 wt.%, achieving enhancements of 47% for γ-Al2O3 and 56% for α-Al2O3 compared to neat epoxy. Numerical simulations reveal that γ-Al₂O₃ induces morphology-dominated field intensification, while α-Al2O3 exhibits orientation-dependent behavior. The introduction of a functionalized interfacial layer significantly reduces peak electric field by up to 53%. Proximity-controlled simulations demonstrate that severe field amplification occurs when the inter-particle gap decreases below 10 nm, indicating that dielectric failure is governed by proximity-critical configurations rather than filler loading alone. These findings establish the minimum inter-particle gap (gmin) as a key parameter linking nanoscale dispersion to macroscopic dielectric reliability.