I vote Sigma-19, given the He-3 would have a reasonable sellpoint like Helium had Erie.
Since GMG is a serious backbone for the overall Sirius-economy and Sigmas itself are meeting place of many factions (such as Cryer, Kruger, Corsairs or Blood Dragons), I'd vote for this being the start point, although with different default restart consisting of GMG ID Garanchou AND similiar He-3 selling place in Sigma-19 such as Planet Erie was for Penny.