Make the filling by folding together 2 ½ cups of the buttercream with ½ cup of lemon curd. Set aside.
Once the cakes have completely cooled, carefully cut each in half (horizontally) with a long, serrated knife to create four, even layers. Generously brush each layer with the thyme syrup.
Place the bottom layer of cake on a cake plate or serving dish. Fill a pastry bag fitted with a medium plain (round) tip with some of the buttercream. Pipe a ring of frosting around the outer top edge of the cake. Fill the ring with about ¾ cup lemon filling. Dollop on a couple tablespoons of lemon curd on top and gently swirl.
Top with the next layer of cake and repeat. Frost the cake with the remaining vanilla bean frosting. Garnish with fresh thyme, if desired.
To create the ombre swirl effect, remove about ⅓ of the remaining buttercream. Tint that portion the desired color of choice and set aside. Top the cake with a large scoop of plain vanilla bean frosting and smooth out, allowing for some of the frosting to get pushed over the sides. Take the tinted frosting and apply it to the bottom ⅓ of the cake with an off-set spatula.
Mix the tinted buttercream with any remaining buttercream to create a lighter shade (add more coloring, if necessary). Fill in the sides of the cake with the lighter frosting. Once the cake is covered in buttercream, smooth out the sides with an icing smoother, off-set spatula, or bench scraper.
For the swirled effect, place the cake on top of a cake turntable. Touch the tip of a metal spatula to the bottom of the cake. Rotate the turntable and move the spatula straight up the side of the cake. For the top, place the tip of the spatula near the outer edge. Rotate the turntable and move the spatula towards the center of the cake to create a swirl.