Leverkusen are on a great run with Xabi Alonso. They can't be underestimated in the Champions League at all. However there is still a huge experience and success gap between Leverkusen and Bayern Munich.
Bayern Munich have a great history with 6 titles here. Leverkusen have only 1 runner-up season and that's all. They have a great momentum recently but experience plays a really important role in the Champions League. I don't think Leverkusen will do better than them honestly but it is just my opinion.
Hansi Flick is a top quality coach at a top level, what's remaining for him would be qualified and quality players who can play the La Ma Sia system.
Barcelona will need time and new transfers to bring back their strong days. Hansi Flick is a great manager choice but the squad should be more competitive while Real Madrid is signing Mbappe.
Regarding Bayer Leverkusen, of course they still have a good chance next season in the Champions League to get good results. At least, qualifying for the last 16 or reaching the quarter-finals would certainly be a good result for Bayer Leverkusen. But indeed, at least because Xabi Alonso still has great abilities, maybe the result could be more likely to qualify for the semifinals, who knows. But regarding Barcelona, basically Barcelona has no problems with their squad and performance.
Because after all, Barcelona's only problem is their poor finances. Of course, because Barcelona's poor finances are difficult to overcome from season to season, Barcelona will continue to have difficulty improving the quality of their team from season to season too. Even though Barcelona succeeded in appointing Hansi Flick as their new manager next season but still, because of Barcelona's poor finances, it will certainly be difficult for Hansi Flick to improve Barcelona's performance.