The most difficult thing may be defending the title because it takes mentality and support for PSG but for Barcelona it may not be easy even though if you look at the number of players they have that are quite supportive. Hansi Flick managed to make Barcelona perform better than the previous season but it took time to achieve the title that the fans wanted. The Champions League looks very competitive and there it is not only about the quality of the players but also other factors, this is what may be difficult to achieve. But optimism is important as long as we see the direction of the changes made by Hansi Flick going well.
Hansi Flick did succeed in giving the best tactics to Barcelona players last season, but they also still need to be given a way to maintain their mentality when playing in the Champions League so that when they are in a losing position while competing in the Champions League, their mentality does not immediately drop. Because it is true as you said that building a steel mentality in each player is indeed not easy and even quite difficult even though the player is indeed very compatible with the tactics applied by his coach.