<p>Improving energy efficiency and reducing resource waste motivate sustainable thermoelectric technologies for harvesting low-grade heat. Yet achieving high-performance and durable systems remains challenging due to coupled transport and multiscale optimization. Here, we establish a scale-bridging interface-centric design strategy that links macroscopic junction engineering and microscopic interface modulation. Based on n-type Mg₃(Sb, Bi)₂ as the representative system and guided by comparative evaluation of device-level contact interfaces, we translate Mg₂Ni into a scale-bridging interfacial phase in n-type Mg₃(Sb, Bi)₂: it forms low-resistance and thermally stable external junctions while, as an internal interfacial modifier, alleviating grain-boundary carrier scattering and strengthening phonon scattering. This scale-bridged interface design yields an average <i>zT</i> of 1.5 over 300 to 723 K with high thermal stability. By applying Mg₂Ni as both an internal modifier and an external barrier layer, a two-pair module delivers a stable conversion efficiency of 8.3% when coupled with p-type MgAgSb, comparing favorably with representative Bi₂Te₃-based thermoelectric modules in terms of conversion efficiency under comparable temperature differences. This work establishes a bridging interface design framework that links device contact engineering with internal interface modulation, providing a practical route toward efficient and durable thermoelectric energy conversion.</p>