This achievement can easily be gained by changing an already started game's world save.
When launching into a saved game, right click the file and there is an option to "Regenerate World".
This will keep your current ship, designed fleet, money and resources, but will give you the ability to generate a new world for the save.
When selecting the options for the new world generation you can lower the world size to 10%.
This makes the completionist achievement much easier to acquire as you go from needing hundreds of sectors cleared to only about a few dozen.