Pardon my inactiveness, I was on a short trip
And yes, after beating the
/!\ Spoiler Warning /!\
Fourth* Boss (Advena Boss, because the ??? Boss, as hard as it is, isn't the final one), you will enter Free Roam mode. My creations basically hinge on a lot of Free Roam farming (enemies spawn infinitely there, along with natural-spawning nutrient pieces), so they can take quite some time to make.