Low-$T_c$ TES based radiation detectors are excellent choices for experiments in fundamental physics such as direct detection of low-mass dark matter, neutrino-less double beta decay search, and coherent neutrino nucleus scattering, owing to their advantages of low threshold, high energy resolution, and fast response time. We have been developing low-$T_c$ materials and devices with the goal of realizing low-$T_c$ TES detectors for various applications in fundamental physics research. In this presentation, we will discuss work carried out in collaboration with UC-Berkeley, to develop large-area low $T_c$ detectors as potential low-threshold light detectors for a neutrino-less double beta decay experiment. We have successfully developed a number of recipes for low-$T_c$ superconductor films including Ir/Pt bilayer and Au/Ir/Au trilayer with tunable and reproducible $T_c$'s down to 20 mK and sharp superconducting transitions. Here we discuss our studies of thermal transport of our materials and present measurements of thermal conductance from both electron-phonon decoupling within our metals and materials interfaces.