hi

I slice my cakes and fill them with a Swiss meringue buttercream filling and then cover them in fondant, but I can always see bumps where I have cut and filled the cake. Do I need to buttercream the sides more? Or is there another way to prevent this?

I would make sure that your dam of icing inside isn't too big, since the weight of the cake and fondant on top of it is probably pushing it out. I always make sure that I do mine a little bit back from the edge so that it has room to spread when I stack the next layer. HTH!

I always set something with a little weight to it on top of my crumb coated and wrapped cake so the frosting can bulge out before I put fondant on. Then I smooth the bulge, recover and it goes in the fridge for a bit to set up.
then put your final coat of buttercream or fondant.
Hope that helped a bit