(K,Na)NbO₃ (KNN)-based ceramics are highly expected to attain a large piezoelectricity (d₃₃), high Curie temperature (T_C) and improved temperature stability. In this study, we modified both d₃₃ and T_C by the construction of phase boundaries using a different chemical composition. As shown in this manuscript, the relationships among the composition, phase boundary and electrical properties can be well established. By designing a series of material systems of 0.95K_0.40Na_0.60NbO₃–0.05Bi_0.5M_0.5HfO₃ {M: Li, (Li, Na), (Li, K), (Li, Ag), (Li, Na, K), (Li, Ag, Na), (Li, Ag, K), (Li, Ag, Na, K), Ag, (Ag, Na), (Ag, K), (Ag, Na, K), Na, K, (Na, K)}, the orthorhombic–tetragonal (O–T) phase boundary can be well established and the enhanced d₃₃ of 330–370 pC N⁻¹ and a high T_C of 308–320 °C can then be attained simultaneously in the ceramics without Li. In particular, there is also an improved temperature stability of d₃₃ in KNN–BAH ceramics, i.e., a high d₃₃ of 200 pC N⁻¹ and d₃₃* of 300 pm V⁻¹ can be shown by the ceramics even if the measurement temperatures reach 150 °C. As a result, a balanced development of three parameters can be well realized in KNN–BMH ceramics and we believe that this material system will give a clear understanding about the comprehensive behaviors of KNN-based ceramics with a high T_C.