The development of high-performance lead-free energy storage capacitors is crucial for sustainable technologies, yet hindered in NaNbO₃-based antiferroelectric (AFE) ceramics because of significant polarization hysteresis from field-induced AFE-ferroelectric (FE) phase transitions. This hysteresis fundamentally limits the simultaneous optimization of recoverable energy density (W_rec) and efficiency (η). Herein, we demonstrate that lamellar nanodomain engineering via compositional design in a (0.87–x)NaNbO₃–0.13Bi_0.5Na_0.5TiO₃–xBi(Mg_0.5Ti_0.5)O₃ system effectively overcomes this bottleneck. The optimized composition (x = 0.05) delivers exceptional energy storage performance with a W_rec of ~8.2 J/cm³, a η of ~88.9%, and a power density of ~207 MW/cm³. Analysis on multiscale structure evolution reveals that this compositional tuning induces a phase transformation from AFE P to AFE R symmetry, accompanied by an enhanced local structural disorder. Critically, the formation of lamellar AFE R-phase nanodomains with width ranging from 2 nm to 6 nm drives a quasi-linear polarization response with minimal hysteresis. Concurrently, the refined grain size improves the ceramic resistivity, substantially enhancing dielectric breakdown strength. These synergistic effects collectively yield outstanding energy storage properties, demonstrating that engineering lamellar AFE R-phase nanodomains is an efficient strategy to optimize overall energy storage performance of NaNbO₃-based materials.