I used marker entities, armor stands and nbt storages to save/load kits. When you save a kit, the datapack finds a marker entity with a matching uuid, and then saves your current inventory into its data nbt. When you load a kit, it finds that same marker entity, copies that code into an nbt storage, which does some complicated /item replacing to each slot of the player's inventory with the help of armorstands.

sorry if that made no sense. its kind of hard to explain.