Half Wall Trims

As far as I know you can't remove the trim, just change it. There are several variations but to me at least, apart from the glass one and the one with the fence on top, they all appear the same. If a space is completely surrounded by walls, fences, spandrels or half walls, it's considered a room by the game.

I also believe the half walls can be added "wall by wall" like the garden fences. So it should be easy replacing a normal wall with half walls, even for a full room. The only problem would be the door - perhaps you must need to use fence+gate for that one tile, or leaving it open (which might make the space not a separate room).

If a half-wall is tall enough (one of the taller half-wall options) you can fit a door from the "short wall" (normal walls) category on it and it functions. It definitely counts as its own room. I use this to make restroom stalls in public buildings. The are counted as separate rooms, so sims don't get embarrassed, but the walls are lower than the main walls, so they look like stalls. The "half-wall" just has to be barely tall enough to fit the door and it works.