Epoxy-based nanocomposites are promising solid insulation materials for high-voltage applications because of their high dielectric strength, mechanical robustness, and processability. However, identifying the optimal nanofiller loading that maximizes dielectric breakdown strength (BDS) remains challenging because conventional trial-and-error approaches are costly, time-consuming, and difficult to generalize across material systems. This study proposes a a cross-material physics-informed machine learning framework integrating four structurally distinct nanofillers: Zn/Al-LDH, Mg/Al-LDH, γ-Al₂O₃, and α-Al₂O₃, where the alumina nanoparticles were synthesized from recycled aluminum beverage cans as a sustainable material source. For each system, 15 breakdown measurements per concentration were statistically validated using Weibull analysis. Among all systems, α-Al₂O₃ exhibited the highest BDS of 46.8 kV/mm at 5 wt.%, corresponding to approximately 56% improvement over neat epoxy. Dataset augmentation was performed using PCHIP interpolation combined with Gaussian noise injection and validated through Leave-One-Out Reconstruction analysis, which showed interpolation errors below 7.76% for internal concentration points. Composite relative permittivity at intermediate concentrations was estimated using the Maxwell–Garnett model and incorporated as a physically constrained input feature. Five regression algorithms were trained and benchmarked on material-specific datasets, achieving R² values up to 0.959 with experimental validation errors below 5.2%. A cross-material model was further developed by replacing categorical material identity with intrinsic filler permittivity as a physics-based descriptor, enabling a transferable across multiple investigated nanofiller systems using a common descriptor. The cross-material model achieved R² = 0.919 with prediction errors below 6%. The proposed framework provides a scalable and experimentally validated route for optimizing epoxy insulation systems for GIS/GIL spacer applications.