I just see it very simple for now. Real Madrid is bringing Mbappe this summer. They are preparing to announce him even before Euro 2024 probably. If Barcelona doesn't have anything to answer then they should be ready to finish the La Liga at the second position once again. 
Hansi Flick isn't magician but he is a great manager. He would have some good touches on the team for sure.
But the squad quality difference would be felt anyway. Real Madrid also has a great manager like Ancelotti in the end. He knows how to deal with such a strong squad. Even after Mbappe I don't think he will have big difficulties in doing that.
If Kylian Mbappe ends up joining Real Madrid, then on paper Barcelona will be far behind their rivals. Hansi Flick isn't magician but he is a great manager. He would have some good touches on the team for sure.But the squad quality difference would be felt anyway. Real Madrid also has a great manager like Ancelotti in the end. He knows how to deal with such a strong squad. Even after Mbappe I don't think he will have big difficulties in doing that.
The depth of Barcelona's squad would not be better if management did not spend a lot of money to buy several players. Barcelona may have to rely on several more young players such as Lamine Yamal, Gavi, Pedri, Cubarsi and Balde next season, so it is clear that the depth of their squad is very inversely proportional to what Real Madrid has.
Barcelona needs more experienced and proven players if they want to win the title, but they can still win the title if their new coach can motivate his players to work harder like Leverkusen in the Bundesliga. Any team can win the title as long as the players have the desire and try to perform optimally in every match throughout the season, that could happen for Barcelona next season.