So the last few times I've added a layer of strawberries on top of the buttercream in between the layers of cake, they've tasted a little old and mushy (not sure how else to describe)... I'm thinking it's because I make my cakes a day or 2 in advance. What's the best way to prevent this? Maybe adding lemon juice to the strawberries? Thanks in advance

I love fresh strawberries but seldom add them between the layers of cake for the exact reason that you state. They get old and mushy and start seeping too quickly. In my opinon, fresh fruit should only be used in cake that is going to be served the same day that it is made.