Understanding the structure–property relationships of materials in order to supress thermal conductivity is crucial for developing efficient thermoelectric generators and thermal barrier coatings. Low thermal conductivity materials can often contain a single dominant phonon scattering mechanism. Here, we highlight how combining different structural features into one material can aid in the design and identification of new materials with low thermal conductivities. We synthesise two new mixed-anion materials, Bi₈CsO₈SeX₇ (X = Cl and Br), with low thermal conductivities of 0.27(2) and 0.22(2) W m⁻¹ K⁻¹ respectively, measured along their c-axes at room temperature. The Bi₈CsO₈SeX₇ materials possess a combination of bond strength hierarchies, Cs⁺ vacancies, and low frequency Cs⁺ rattling. These different features significantly inhibit phonon transport along different crystallographic directions. Due to sharp bond strength contrast between the van der Waals gaps and [Bi₂O₂]²⁺ layers, the Bi₈CsO₈SeX₇ materials exhibit thermal conductivities <50% of the theoretical minimum when measured along the stacking direction. Conversely, the thermal conductivity associated with the ab-plane is reduced by Cs⁺ rattling when compared to the structurally and compositionally related BiOCl.