Additive manufacturing technologies in construction, particularly concrete 3D printing, offer new opportunities to automate building processes and improve material efficiency. However, the use of agricultural waste in cement-based composites for 3D printing remains limited, and the combined effects of such waste on rheology, interlayer bonding, mechanical performance, and thermal insulation are still insufficiently studied. This study aims to develop 3D-printable cement-based composites incorporating ground rice husk (GRH) as a multifunctional filler. GRH was introduced at dosages of 0–40% by mass of cement while maintaining constant workability. Because GRH is hydrophilic and water-absorbing, increasing its content required progressively higher amounts of added mixing water. Consequently, GRH dosage and water adjustment were not independent variables, and the reported effects represent the combined response of the composite to GRH incorporation and the associated water adjustment required to preserve printability. Under these conditions, increasing the GRH content reduced the compressive strength from 45.6 to 5.2 MPa, the flexural strength from 8.0 to 1.9 MPa, and the interlayer bond strength from 0.56 to 0.13 MPa, while thermal conductivity decreased from 0.85 to 0.35 W/(m·K) and water absorption increased by 98%. SEM confirmed the transition from a dense cementitious matrix to a highly porous structure, with filler particles connected by thin films of cement paste. The study demonstrates the potential of using rice husk in 3D-printable cement-based composites and highlights the trade-off between mechanical performance and thermal insulation as GRH content increases.