This study investigated maximizing alumina filler loading in epoxy by comparing aromatic and aliphatic surface modifications. The aromatic treatment outperformed the aliphatic alternative, achieving a 52.6% reduction in viscosity compared to just 3.2% for aliphatic, proving that aromatic–aromatic interactions significantly enhance filler-matrix compatibility. These findings were supported by spectroscopic analysis and resulted in composites with 20.6% higher thermal conductivity, increased mechanical stiffness, and a lower coefficient of thermal expansion. Practical thermal testing on an LED-mounted PCB confirmed that the aromatic modified system reached a 3.05°C higher steady-state temperature than unmodified fillers, making it an ideal candidate for high-performance, thermally conductive adhesives.