I don’t think you can directly reference a level blueprint. But you can add blueprint interfaces to reference/call functions on these level blueprints.
Have an array of Blueprint Interfaces “BPI_LevelBP” where your levels have access to.(GameState/GameInstance/PlayerController?) Maybe 1 Ref for main level and 1 array of Refs for sublevels. And on each level’s BeginPlay & EndPlay do add & remove to the array. Implement the interfaces so that your levels can communicate through them.