Reversible solid-state thermal switches with large thermal contrasts remain rare, yet are essential for advanced thermal management. Using universal machine-learned interatomic potentials (uMLIPs) and density functional theory (DFT), we screen 57 transition metal dichalcogenides (TMDCs), spanning the 1T and 2H phases with Li and Mg intercalation. Among the candidates, LiₐZrS₂ combines exceptional thermal conductivity switching with low heat capacity. Lithiation to Li_(0.125)ZrS₂ reduces the cross-plane thermal conductivity from approximately 3.7 to 0.05 W m⁻¹ K⁻¹. The switching behaviour exceeds that of previously reported solid-state electrochemical thermal switches. Device-level modelling further demonstrates the potential for waste-heat recovery, with a pulsed system capturing 86% of the energy recovered by an ideal switch. We attribute the switching behaviour to substantial softening of the Zr–Zr framework, with the mean force constant decreasing from approximately 4.2 to 1.2 eV mathringA⁻², alongside enhanced scattering of heat-carrying acoustic phonons. These findings establish electrochemical intercalation as a promising route to engineering thermal transport in layered materials.