High-frequency core loss (P_cv) constitutes a critical bottleneck restricting the widespread application of soft magnetic composites (SMCs) in next-generation, high-frequency, miniaturized power electronic systems. To effectively suppress such losses, the synergistic optimization of insulating coatings and processing parameters represents a core strategic approach. In this work, Fe_73.5Si_13.5B₉Nb₃Cu₁ nanocrystalline powders with a mean particle size of 12.98 μm are employed as the magnetic matrix. A series of SMC toroidal cores with varying epoxy resin (EP) coating contents is then fabricated through a sequential process of powder insulation, uniaxial compaction at 1800 MPa, and subsequent stress-relief annealing at 450 ℃. The effect of EP coating content on the insulation integrity and high-frequency magnetic performance of SMCs is systematically investigated. At an optimal content of 2 wt.% (designated as EP2), the P_cv is significantly reduced, yielding values of 628 mW/cm³ at 20 mT/1000 kHz and 283 mW/cm³ at 50 mT/100 kHz. These loss characteristics are superior to those of most reported SMCs under equivalent testing conditions. Bertotti loss separation reveals that increasing the EP coating content slightly reduces eddy-current loss (P_e) while substantially increasing hysteresis loss (P_h). Specifically, upon increasing the EP content from 2 wt.% to 3 wt.%, the contribution of P_e to P_cv decreases from 60% to 48%, whereas that of P_h increases from 40% to 52%. This indicates a transition of the dominant loss mechanism from P_e to P_h, with the rising proportion of the latter being the primary cause for the overall increase in P_cv. The proper EP coating content reduces porosity and enhances the densification of the magnetic powder core. Meanwhile, it forms a uniform, continuous, and dense coating layer that effectively suppresses localized agglomeration between particles. This combined effect enables the EP2 sample to achieve an optimal balance between P_h and P_e, thereby yielding the minimum P_cv. Through precise control of the EP coating content, synergistic regulation of individual loss components can be realized without resorting to complex strategies such as multi-layer coating or interfacial modification. This approach thus provides an effective and straightforward pathway for loss management in high-frequency, high-power magnetic components.