@eufl I had no problem with friezes using the method I described above, either with platforms or decks( those are the ones you are referring to), however, I did not try anything too elaborate. I never 'exploded' the room, since there was no need, maybe that's why the friezes don't show up on the inside?
You are wrong about one thing though, you actually CAN paint the platforms any wallpaper you want ;) Just surround the platform with a "half wall", make sure this half wall is only the size of your platform, not higher, and your stairs will still be usable! I have used this method a few times like this, thoroughly tested it and it poses no problems considering functionality. The only part that cannot be painted at will is then the half wall trim, but I found that to be less annoying than the entire walls of the platform.
https://i.imgur.com/0EuWHlN.png
This method does not work when you use the platforms by themselves as 'stairs', the walls, without staircase, will prevent sims from stepping on them, you need to place a staircase in front.