How to load sub-sub level recursively?

I’m creating a stage transition system from Stage1 contains some sub levels to Stage2 contains some sub levels, and from Stage2 to Stage3.

But sub level number of each stage is variable (1 sub level ~ 5 sub levels.) However, each stage is also Persitent Level’s sub level, it cannot have sub-sub levels.

If I load Stage1, It seems to call “Load Stream Level” from Persistent Level many times with “For Each”, the number of Stage1 and Stage1’s sub levels.
I would like to do it EFFICIENTLY, I would like to load Stage1 such as a parent of tree with sub levels as children recursively.

Can I access sub level’s hierarchy on BP or C++ and load sub-sub levels manually?