Hello! I find this of interest as I am the creator of that other thread. Later in the thread, I actually came to the same conclusion you started with - premade rooms (at least, in terms of their walls) are simply easier to work with and faster to process.
In any case, I’m trying to get an idea as to what you’re asking. So let’s say you have a “kitchen” room. It has 4 doors, North, East, South, and West. The center of the room has a table with chairs, the NorthEast corner has a water basin and the SouthWest corner has a shelf with cooking supplies.
You want to create this entire room so that it is treated as a single actor? I would create a “kitchen” blueprint and make as much as you could in the components tab. The only thing I think you wouldn’t be able to add there is other blueprints (unless I’m mistaken in which case, that’s your answer). I would add a vector point at each location you wish to spawn a new blueprint (maybe the shelf is a separate container blueprint).
Then, go into the event graph and make a “Begin” event node and “Spawn actor from class”, using one of your placed vector points as the location. Then, I believe the function is “Attach actor to actor”, attaching the spawned actor to your room.
After that, I believe any time you rotate the Kitchen actor, all the other attached actors will maintain their relative transform within the kitchen as well.
If this is not what you’re asking… sorry! Please re-phrase and I’ll try again 
I’m at work, I’m bored, I’ve had lots of coffee.
Edit: One other thought. You can change the transform of an actor as you spawn it, removing the need to rotate it after the fact. Unless rotating the room is part of the game which admittedly sounds pretty awesome.