(1 − x)Bi_0.5(Na_0.80K_0.20)_0.5TiO₃–x(K_0.5Na_0.5)MO₃ (M = Sb, Ta) (BNKT20–KNM100x) lead-free piezoelectric ceramics were designed and fabricated using a conventional fabrication process to achieve large strain response in BKNT20-based ceramics. The KNM substitution was found to induce a transition from ferroelectric to relaxor pseudocubic phase, and such transition is accompanied with the significant disruption of ferroelectric order and the shift of the ferroelectric–relaxor transition temperature T_F−R down to room temperature. Accordingly, large electric-field-induced strains of 0.39–0.41% (at 80 kV cm⁻¹, equivalently 488–513 pm⁻¹ V), which are derived from a reversible field-induced ergodic relaxor to ferroelectric phase transformation, were obtained in 1.25 mol% KNM-modified compositions near the phase boundary. Moreover, an attractive property for application as actuators was obtained in the present system, compositions near the phase boundary with an ergodic relaxor state exhibited fatigue-free behavior after 10⁶ cycles. Furthermore, unexpected almost fatigue-free behavior was also observed in 0.5 mol% KNM-modified samples with a typical ferroelectric long-range order. Results of the enhanced activation energy (E_a) for electrical conduction suggest the well-observed fatigue-resistant behavior in the present system should be mainly attributed to the lower defect density. These findings give the current material great opportunity for actuator applications demanding improved cycling reliabilities.