If he's already the nation's strongest and soon world's strongest, then who's going to be the enemy here? Could there ever be a threat? There will be no tension at all, notice how other op isekai MC's that are loved (for example Rimuru and Ainz) are op but there are still someone stronger than the mc, in Ainz case, he wasn't even the strongest in his own guild (before he got isekai'd) he's not even the strongest in the new world, hell some of his subordinates are stronger than him. In Rimuru's case, sure he will be the strongest but that was at the very end of the series, but at the beginning it was stated that there are demon lords, true dragons (like veldora), and other worlder like him that are heroes who are much more powerful than Rimuru meaning that there are threats.