The applications of piezoelectric materials for electromechanical conversion devices require both high piezoelectric response and excellent performance stability. In this work, BaHfO3 and (Bi0.5Na0.5)ZrO3 modified (K0.5Na0.5)(Nb0.96Sb0.04)O3 ceramics with high piezoelectric response (d33~490 ± 20 pC/N, d33*~860 pm/V) and superior thermal stability (variation of d33* is below 12% over the temperature range of 25–180 °C) as well as cyclic reliability were achieved by integrating the strategies of phase boundary design and crystallographic texturing. The high piezoelectric response originated from three synergistic effects of R-T phase coexistence, piezoelectric anisotropy, and nanodomain configuration. Based on the in-situ temperature dependence of XRD and TEM analyses, we disclosed that the stable R-T phase coexistence and domain morphology over a wide temperature range contributed to the excellent thermal stability of piezoelectric properties. This work not only provides an effective strategy to simultaneously achieve high piezoelectricity and superior temperature stability, but also uncovers the structural origins responsible for the stable output in KNN-based ceramics.