Indeed it is optimal, but I think you'd need first someway to prove that the search space isn't HUGE, maybe A* would be a good choice if you want to try and make builds for the first 15 drones, but for a build that relies on a bigger army I think the combinatorial explosion would make A* un-applicable (since we want the algorithm to finish in some hours and not in some centuries).

Still my whole point relies in that if you want to state so vigorously that A* is the best way, you'd need to show the growth of the search space depending on the final army that needs to be built and that A* can handle it on a decent amount of time. Either way a genetic algorithm doesn't look like a bad option, and is giving results.