Shell-based modeling of Triply Periodic Minimal Surface (TPMS) lattices enables efficient simulation and optimization of architected materials with complex geometry. Recent studies have demonstrated that redistributing the local shell thickness on a fixed midsurface is an effective strategy to tailor stiffness and elastic anisotropy while preserving periodicity and manufacturability. However, standard shell formulations inherently enforce symmetric thickness distributions about the midsurface, which limits control over bending leverage, membrane–bending coupling, and the spatial placement of material relative to curvature. This contribution proposes a sequential size–shape optimization framework for TPMS shell lattices that addresses this intrinsic modeling limitation. In a first stage, a sizing ptimization redistributes the local shell thickness on a fixed midsurface using periodic boundary conditions and homogenization-based stiffness evaluation. Once thickness optimization converges, a second stage introduces controlled midsurface relocation by displacing the shell geometry along its normal direction. This shape update effectively shifts the material centroid in space and enables asymmetric material placement that cannot be represented by shell thickness variation alone. While shape optimization of TPMS lattices has been explored previously, the present work focuses on shell-based lattices and explicitly couples thickness redistribution with controlled midsurface relocation. The proposed strategy preserves the computational efficiency of shell-based homogenization while introducing an additional geometrical degree of freedom with a clear mechanical terpretation. Numerical studies on representative TPMS unit cells indicate that midsurface relocation can yield further improvements in effective stiffness and significantly influence the anisotropic elastic response beyond what is achievable through thickness optimization alone. The results highlight midsurface positioning as a previously underexplored design variable in TPMS shell lattice mechanics and provide a systematic route toward enhanced performance through coupled size–shape optimization.