My brother did a kingdom for me, the luncheon kingdom and I want to do it myself so is there a way I can delete the progress just for 1 kingdom?
Completing a Kingdom changes the Map a bit. So if you completed it once, you cannot play it like it was in the beginning.
Unfortunately you cannot replay a Kindom once it is completed. But you can create multiple accounts on your Nintendo Switch. And every time you start the game (after closing it completely) you can select the created user. Each of them has its own save slot. So you can replay it as a different user from scratch.