Hello,
There are a few delegates that you can register to so that they run after a level was loaded. That would allow you to iterate over the actors in the new level.
Look for FEditorDelegates::OnMapOpened and FWorldDelegates::LevelAddedToWorld as a starting point. There is also ULevelEditorSubsystem that “shadows” some of the delegates to make them accessible through BP or Python.
There are a lot of delegates declared in the Editor.h. You should be able to find one that works for your case if the ones listed are not firing at the right time.
Regards,
Martin