Since there is no current way to reset the campaign normally, there is one other way to do it while keeping your character.

First, enter your Roaming folder (type %appdata% in the 'run' console), then the Secrets of Grindea files. Next, the "worlds" folder will have all the world data for each character. World 0 correlates to the first character slot, up to world 8 and the last character slot. To reset the campaign, first go to the starting house in the campaign and quit the game. Then either move, rename, or delete the corresponding world file.

After that, the next time starting the character, the quests will be reset.