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Not a guide as such, more of a tip.
I've heard word that when you reach Jotunheim make sure to get the 2 wealth collectables as this achievement MAY
The missable wealth collectables seem to be during chapter 2 of the Jotunheim story arc. You gain access to an area that will become unavailable once the mission is finished. When you get to this moment in the game you may want to make a manual save before starting.
Apart from that all areas are accessible in post-game.