In conclusion, our investigation systematically explored the intrinsic thermoelectric properties of Sb2Si2Te6 and Sc2Si2Te6, employing a combination of semiclassical Boltzmann transport theory and first-principles calculations. While both layered compounds exhibit low intrinsic lattice thermal conductivities, the low mass difference between Sb and Te results in greater phonon-phonon scattering and shorter phonon lifetimes in Sb2Si2Te6, yielding very low lattice thermal conductivity. We find substantial differences in the electronic structures of these two compounds due to the different orbital characters of the A-site cations. Specifically, Sb2Si2Te6 manifests a small direct bandgap, while Sc2Si2Te6 exhibits a larger, indirect bandgap. The combination of crystal (hexagonal ABC stacking) and orbital (hybridized dx 2 -y 2 , dxz and dz 2 ) symmetry at the Sc2Si2Te6 CBM gives rise to a non-high-symmetry band-edge kpoint and dramatically enhanced band degeneracy (increasing from 1 in Sb2Si2Te6 to ). Combined with good electron mobility in the in-plane direction, this yields an ultrahigh power factor (19 mW m -1 K -2 ) and thus, along with the low lattice thermal conductivity, a large predicted ZT of 3.51 for n-type Sc2Si2Te6 (compared to the optimal ZT of 2.76 for p-type Sb2Si2Te6). This enhanced ZT arises despite the ~3x increase in lattice thermal conductivity upon substituting Sb with Sc, as the band degeneracy enhancement of the PF overcomes this loss. We note that this optimal ZT requires a relatively high charge carrier concentration (n ~ 10 20 cm -3 ), so understanding the defect (dopant) chemistry and possible engineering strategies will be key to realising this performance experimentally. This study demonstrates the potential of Sc2Si2Te6 for thermoelectric applications and, more broadly, the power of band (degeneracy) engineering as a route to designing high-performance thermoelectric materials.