Bi₁₂Rh₃Ag₆I₉ extends the Bi₁₄Rh₃I₉ family of layered bismuth-rich iodides in which intermetallic topological-insulator sheets alternate with salt-like spacer layers. High-temperature synthesis yields phase-pure powders and millimeter-sized platelets. Single-crystal X-ray diffraction reveals a rhombohedral structure composed of [Bi₁₂Rh₃I]²⁺ layers and [Ag₆I₈]^2– spacers. In this spacer, Ag⁺ cations occupy 75% of the tetrahedral voids of a double iodide layer, giving rise to intrinsic two-dimensional cation disorder and mobility. Electrochemical impedance spectroscopy reveals a room-temperature ionic conductivity of about 10^–6 S cm^–1. Density-functional theory including spin–orbit coupling predicts band inversion and weak-topological-insulator invariants, while angle-resolved photoemission spectroscopy corroborates the calculated bulk electronic structure, yielding an experimental bulk band gap of 286 meV. Transport measurements show strongly anisotropic, initially n-type conduction. During the initial measurement cycle, applied bias reduces the electron doping through slow electrochemical deintercalation of silver. After conditioning, bulk conduction is strongly reduced; transport below about 130 K becomes nonactivated, the bias-induced drift ceases below about 50 K as Ag⁺ motion freezes, and a pronounced resistivity drop perpendicular to the layers below 3.2 K is suggestive of a boundary-sensitive low-dimensional transport regime. Bi₁₂Rh₃Ag₆I₉ demonstrates that spacer chemistry provides a chemical lever for carrier-density control in layered topological insulators.