High-performance piezoelectric ceramics are essential for actuators functioning under extreme conditions, ranging from specific electronic components in the aerospace sector to those in the energy exploration industry that operate under moderate thermal conditions. We synthesize a series of 0.5 Pb(Ni_1/3Nb_2/3)O₃-0.5 Pb(Zr_xTi_1−x)O₃ (PNN-PZTx) compositions (x = 0.29, 0.31, 0.33, 0.35) and tune their electromechanical behavior by adjusting the Zr-to-Ti ratio. X-ray diffraction shows a progressive reduction in the splitting of tetragonal reflections with increasing Zr content, placing the x = 0.33 composition near a tetragonal–pseudocubic phase transition rather than a conventional morphotropic phase boundary. This composition exhibits a strongly enhanced electromechanical response, with a quasi-static piezoelectric coefficient d₃₃ of 1054 pC/N, an electromechanical coupling factor of 55%, and a converse piezoelectric coefficient d₃₃^* of 1138 pm/V. An electrostrain of 0.111% is generated at 10 kV/cm, demonstrating efficient actuation at modest fields. Temperature-dependent measurements show that d₃₃ remains within ±20% between 30 and 90 ℃, indicating the coexistence of high activity and thermal robustness. These results demonstrate that precise regulation of the Zr-to-Ti ratio enables access to tetragonal-pseudocubic phase boundaries and provides a simple route to high electromechanical performance in lead-based piezoelectrics for demanding applications.