I tried every trick in the book without success- until I discovered Swiss meringue buttercream. Changed my caking world :) smooth side, flat tops, , sharp corners are a DREAM! Not to mention how well it holds up under fondant.
If you haven’t made it already, Sweetapolita offers the best tutorial. It’s a bit tricky and time consuming to make, but a few dozen (or hundred…) batches later, I can make it in my sleep. Feel free to message me here or on facebook if you have any q’s. Good luck!