Activating a datapack (in part or in full) by an achievement

Hi, I'm trying to make a datapack that changes the overworld spawns for a modpack I'm working on once a player goes to the nether. The idea is similar to how the wall of flesh activates "hardmode" in terraria, stronger and more dangerous mobs would spawn in the overworld following the players first journey to the nether. Is this possible? IDK how to make it trigger like this, but it seems like something that should/would.could be doable with the datapack system. Thanks!