La₂O₃ doped (Na_0.495K_0.455Li_0.05)(Nb_0.95Ta_0.05)O₃ ceramics are prepared using modified milling process, and the influences of La₂O₃ on ferroelectric behaviour, ageing characteristics, thermal stability, electrical stability, crystal structure, microstructure, dielectric and piezoelectric properties were reported. La₂O₃ addition improved the ferroelectric characteristic substantially, and obtained remnant polarization (P_r) and maximum strain (S_max) around 34.3 C/cm² and 0.13% respectively. La₂O₃ doped ceramics improved the thermal stability and were stable up to 180 ℃ compared to undoped ceramics (120 ℃). The Rietveld refinement along with the high-temperature X-ray diffraction studies suggested the presence of monoclinic phase in La doped compositions, which is responsible for their idiosyncratic behaviour. The maximum values were obtained around 179 pC/N and 0.385 for piezoelectric constant (d₃₃) and electromechanical coupling factor (k_p) respectively in La₂O₃ doped samples (0.02 wt%), which also exhibited the lowest ageing rate and stable electrical fatigue behaviour.