You won't want to fully assemble the shortcakes ahead of time since it might get soggy, but you can make all the components separately. Make the strawberry-rhubarb compote and store it in an airtight container in the fridge until you're ready to use (will last up to 2 weeks). For the cake, you can either bake it the day before or make it way ahead of time and freeze it. If you freeze the cake, wait until the cake has fully cooled, then wrap it in a layer of plastic wrap, then in a layer of tin foil. Freeze for up to one month. In terms of the cream toppings, I would recommend making it on the spot. Whipped cream tastes much better when it's fresh. Luckily, this step only takes a few minutes.