If the protagonist was not born with asthma, then he would probably live a better life. However at that point is it even the same person/character if such a fundamental thing is changed. If they are significantly different there is a chance for the world to go through a period of crisis. If I go...
Using my current knowledge I transmute the isekai into a detective novel. Out meta-ing the time magic, while simultaneously nullifying it unless the princess is an expert in planar alchemy.