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Climate warming has profoundly impacted the cryosphere of the eastern Himalaya, leading to rapid glacier retreat, the formation and expansion of proglacial lakes, and the degradation of alpine permafrost. This study examines how proglacial lakes influence the dynamics of both glaciers and rock glaciers (ice-rich debris) in the Sikkim Himalaya, with special emphasis on South Lhonak Lake. We integrate multi-temporal remote sensing datasets – including DEM differencing (TanDEM-X), optical imagery (PlanetScope), and SAR interferometry (Sentinel-1 SBAS InSAR) – to quantify landscape changes from 2016 to 2025. Results show that South Lhonak Lake expanded by ~45% in area (from ~1.12 to 1.63 km²) between 2016 and late 2023 (Qu et al., 2025), before a sudden glacial lake outburst flood (GLOF) in October 2023 drained nearly half of its volume. Pre- and post-GLOF analyses reveal accelerated glacier retreat and pronounced surface lowering at the glacier terminus adjacent to the lake. Rock glaciers in contact with the lake experienced greater surface elevation loss and deformation compared to those in non-lake settings, suggesting that thermal and mechanical erosion by the lake has exacerbated permafrost degradation. Time-series deformation mapping using SBAS-InSAR captured ongoing ground motion on periglacial slopes and rock glaciers, with line-of-sight displacement rates on the order of several cm/year in active zones. Notably, slopes fringing South Lhonak Lake showed progressive subsidence and destabilization signals before the 2023 GLOF, indicative of creeping movement in ice-rich moraine and permafrost materials. These findings highlight a coupling between proglacial lake evolution and the stability of surrounding cryospheric landforms. The study demonstrates the value of synergistic remote sensing for hazard monitoring in inaccessible high-mountain environments. It provides a first regional assessment of how an expanding (and abruptly draining) lake can influence glacier mass loss, rock glacier kinematics, and permafrost stability. Our multi-sensor approach offers a template for early detection of glacial lake outburst precursors and periglacial slope failures, information that is critical for climate change adaptation and disaster risk reduction in the Himalaya.