Hey thanks for using the application and I am glad that you liked it. I understand your requirement and I would love to help you create a solution but for now, the boards are ment to be use and throw and for each retro you should be creating a new board, boards are retained so that in future one can go back and review previous retros and going forward this would become base for people to have all the boards at on place, right now because I don't have any user signup ad login I am not able to deliver that feature but going forward I would need to add it, because that restricts a lot of features. Plus for your problem I had introduced templates where once you create a template you can reuse the same template again and just create a new board for it, without having to add /modify column
Plus you were anyways clearing all the cards, so might as well create a new board as well as it doesn't take much action to do it