Lightweight β-Li₂TiO₃ ceramics are promising microwave dielectrics for the large-scale deployment of 5.5G extremely large antenna arrays (ELAAs). However, their practical application is hindered by the limited Q×f value (where quality factor Q = 1/dielectric loss (tanδ), and f represents the resonant frequency) and large positive temperature coefficient of resonant frequency (TCF), while the underlying phase transition and regulatory mechanisms remain elusive. Here, a unique monoclinic–cubic dual-phase architecture is constructed in Li₂Ti_1−x(Sc_1/2Nb_1/2)_xO₃ (LTSN_x, 0 ≤ x ≤ 0.5) ceramics. In situ structural characterizations reveal that the substitution-induced reduction in the phase transition temperature (T_c) and concomitant lattice distortion synergistically impede the reversed-phase transformation, stabilizing the high-temperature cubic phase at room temperature. Consequently, the inherent negative TCF of the cubic phase effectively compensates for the positive value of the monoclinic matrix, achieving exceptional temperature stability. Furthermore, the reconstructed superlattices and suppression of lattice defects significantly minimize dielectric loss. Of particular importance is that the studied LTSN_0.25 ceramic exhibits excellent microwave dielectric properties, featuring a relative permittivity (ε_r) of 18.6, ultra-high Q×f of 102,330 GHz (at 7.76 GHz), and a near-zero TCF of −2.3 ppm/°C. Simultaneously, the stable THz response and simulated filter performance confirm its great potential for 5.5G applications.