Given that low group velocities are favoured by non-uniform bonding typical of large unit cells and low-symmetry structures, whereas strong anharmonicity is favoured by symmetric tetrel environments typical of high-symmetry structures, the κ κ κ in the chalcogenides is in principle a balance of two competing requirements. However, some chalcogenide structure types appear to "decouple" these factors. The Cmcm phase is a low-symmetry structure, but the tetrel atoms are confined to locally-symmetric environments, resulting in an optimal trade-off between relatively low group velocity and strong anharmonicity. The π phase is a high-symmetry structure, but with a large primitive cell in which the tetrel atoms can adopt distorted local geometries, and based on our findings we would anticipate ultra-low group velocities compensating for weak anharmonicity, potentially representing the other "extreme". Exploring crystal-engineering routes to obtain the group IV-VI chalcogenides in different crystal phases is therefore likely to be a fruitful avenue for optimising the lattice thermal conductivity for thermoelectric applications.