From the wiki:
On your first playthrough of the game, you will find blueprints up to the Sky equipment (unless the player opens a Golden chest in the secret room activated by Calypso's Compass, which can give higher-level equipment), but the best equipment can only be found in the Dungeon or by defeating mini-bosses. In NG+, after beating the final boss for the first time, all of the equipment save the dark equipment becomes available; you will mostly find the new equipment in chests, but will rarely find blueprints for older equipment instead, especially once you have found all the new equipment.
The blueprints for the Dark equipment can be found after defeating the final boss in NG+ by fighting mini-bosses such as Botis, Barbatos & Amon, Stolas & Focalor, and Sallos. They can also be found in NG+ using Calypso's Compass.
I'm on my first playthrough, I already defeated the four bosses but didn't touch the final boss yet because I didn't have all pieces of Sky equipment yet, and since I'd like to get the all equipment achievement I was worried that - as per the wikia - advancing to the second playthrough would prevent me from doing that.
However, even before I completed the Sky equipment, I started getting Retribution equipment from The Land of Darkness! Even more, now I got a piece of Imperial equipment from a regular chest in The Land of Darkness! I'm confused because this is contrary to what is said by the wikia.
When can I safely defeat the final boss and advance to the second playthrough without being worried that this will prevent me from getting some lower level pieces of equipment?