I have seen that you can carve any cake if you freeze it first but what about when it comes to room temperature with the heavy fondant on top of it? I have had a cake start to sag? Do you think I might have put too much buttercream in the middle? Can you limit the amount of filling in carved cakes?
it depends. to thick a layer of filling will bulge in any cake. a bc dam will help to stop this, so if you want a thick layer of filling, then carve the cake unfilled, go back and damn and fill.
light fluffly cakes struggle to support the weight of fondant, so you do need to choose carefully.