An increasing demand for sustainable, high-performance insulation materials in microelectronics and energy systems has accelerated the development of bio-based alternatives to conventional phenol-derived thermosets. In this study, two hybrid polybenzoxazine (PBZ) systems, viz., poly(IMP-BZ/BF-a) and poly(IMP-BZ/C-fa) were developed using an imidazole-cored bisphenol based benzoxazine (IMP-BZ) framework combined with benzoxazine monomers derived from bisphenol-F and cardanol, respectively. The imidazole core, rich in nitrogen and aromatic content, imparts inherent thermal stability, rigid backbone architecture, and reduced chain mobility, contributing to significant reduction in dielectric behavior. To further enhance multifunctional performance, both systems were reinforced with functionalized bio-silica (BS). Among the two hybrid blend systems developed, the cardanol-based hybrid poly(IMP-BZ/C-fa)/BS exhibited superior dielectric properties (k = 2.6, tan δ = 0.007), outstanding thermal stability (char yield > 46% at 850 °C), enhanced flame retardancy, and high hydrophobicity (water contact angle > 141°). These improvements stem from the synergistic interaction occurred between the imidazole core, the aliphatic-rich C-fa structure, and the well-dispersed GPTMS functionalized bio-silica, which collectively restrict dipolar relaxation and enhance network density. This present work demonstrates the potential of imidazole-cored, cardanol-derived PBZ hybrids as sustainable, high-performance materials for next-generation electronic packaging, flame-resistant coatings, and insulation applications.