Despite the fact that relaxor ferroelectrics (RFEs) have been extensively researched because of their various advantages, there are still barriers to simultaneously increasing their energy storage density (W_rec) and efficiency (η). By substituting Bi(Mg_0.5Sn_0.5)O₃ (BMS) and optimizing the formation process, this study follows a stepwise optimization route to achieve comprehensive exceptional energy storage performance (ESP) in Na_0.5Bi_0.5TiO₃-Sr_0.85Bi_0.1TiO₃ (NBT-SBT)-based ceramics. On the premise of constructing a Sr²⁺–Sr²⁺ ion pair at the A-site to ensure a large polarization, the introduction of Mg²⁺ and Sn⁴⁺ ions at the B-site further induces a local disordered field and promotes polar nanoregions. Following that, the viscous polymer process (VPP) used to synthesize NBT-SBT-BMS ceramics can thin the thickness, reduce defects, and boost compactness, hence improving the polarization difference (ΔP) and breakdown strength (E_b). Using the stepwise optimization route, we were able to attain a high ΔP of 64.6 μC cm⁻² and an E_b of 440 kV cm⁻¹ in 0.92(0.65NBT-0.35SBT)-0.08BMS-VPP ceramics. More crucially, an ultrahigh W_rec of 7.5 J cm⁻³ and a high η of 85% are simultaneously achieved, together with excellent temperature adaptability between 20 and 120 °C. Our superb ESP exceeds the majority of previously reported NBT-based ceramics, confirming the applicability of this stepwise optimization route to other similar high-performance dielectric ceramic designs.