Laser Powder Bed Fusion (LPBF) research on aluminum alloys has predominantly focused on commercial Al–Si–Mg systems with modest Mg ₂ Si contents, leaving the heat-treatment response of Mg ₂ Si-rich compositions largely unexplored. This study investigates, for the first time, the hardness response of an LPBF-tailored aluminum alloy containing 3.94 ma% Mg ₂ Si—over twice the equilibrium solid-state solubility limit—and identifies an optimized post-processing route to enhance mechanical performance. Three heat-treatment conditions were investigated: direct aging, solution annealing (SA), and combined solution annealing plus artificial aging (SA + A). Hardness mapping revealed that direct aging at 190 °C for 330 min achieved the highest hardness of 164 HBW2.5/67.5. The as-built and direct aged conditions were investigated using SEM, EBSD, and TEM to provide a comparative perspective of the microstructural changes that explain the increase in specimen hardness between the two conditions. The as-built state revealed a highly supersaturated α-Al matrix concentrated with early stage Al-, Mg- rich clusters and a refined Si network. In the direct aged condition, nanoscale precipitates were observed, consistent with the precipitation of Mg- and Si-rich phases responsible for the hardening response. The results indicate that the supersaturated solid solution generated during LPBF processing renders a solution-annealing step unnecessary, offering significant reductions in energy consumption. By correlating the heat-treatment routes with the resulting hardness and investigating the increase in hardness by microstructural investigations, this work provides a comprehensive comparison of post-processing routes and offers new insights into designing efficient, and sustainable thermal treatments for high performance aluminum alloys.