I don't think it'll be too hard; you can build infinite legendary iron from just a dozen or so foundries on Vulcanus. Legendary coal and stone are not much harder, and from legendary coal you get legendary plastic and then legendary steel and copper via the LDS shuffle. It's not quite as broken as the asteroid casinos and feels much more factorio-y IMO.

Avadii made a great guide about good ways to farm legendaries in 2.1: https://www.youtube.com/watch?v=ZuwBnOsULkc