The growing demand for low-carbon building materials and the challenges of handling agroforestry waste and discarded EPS particles have spurred research toward developing novel building composites that utilize solid waste. Therefore, this study aims to develop a lightweight geopolymer composite incorporating these recycled materials to balance thermal insulation, mechanical strength, and waterproofing properties. In this work, geopolymer served as the binder, with various types of raw biomass (sawdust, rice husk, rice straw, and coconut fiber) as the primary aggregates and EPS particles as an additive to create a closed-pore structure. The microstructure of the raw biomass was characterized by SEM, while its specific surface area and average pore diameter were determined by BET analysis. Furthermore, the prepared composites were comprehensively evaluated in terms of their microstructure, pore structure (MIP), density, thermal conductivity, compressive strength, total water absorption, capillary water absorption, surface wettability, and UV aging behavior. The results showed that the prepared composites exhibited a porosity of 59.9–65.7%, a density of 492.9–586.3 kg/m³, a compressive strength of 7.3–10.9 MPa, a thermal conductivity of 0.115–0.142 W/(m·K), a total water absorption of 35.2–42.2%, capillary water uptake coefficients of 4.9–11 kg/m², and a water contact angle exceeding 140° (after modification). In addition, the developed composites offered significant environmental and economic benefits, with a low carbon footprint and an estimated cost of 100.6–150.3 USD/m³, making them more competitive compared to traditional insulation materials. Meanwhile, this study provides a scientific basis for developing high-strength building insulation materials from agroforestry waste, thus outlining a promising direction for future research and industry development.