What a cute design, love your colors. I would say, that since it's so difficult to pipe over all the decor on top, utilize your cake board. The cake board is a great place to really take your cakes over the top. You can decorate them with fondant, icing, paper, ribbon, etc. And, you can use them for the writing as well.
I agree with using the cakeboard and also use a piece of wax paper to practice your piping handwriting. It's not easy, and it does take some practice to be confident with it on the actual cake. Also, use some sort of straight edge tool to ice the sides of the cake to make it a smooth surface. Give the cake a crumb coat or dirty ice as some say first, doesn't have to be pretty. Then refrigerate it for a little while to harden that layer of icing. Then put a second coat of icing which is the layer you want to make smooth.
Best of luck!
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