Leverkusen only needs experience to continue to strive to continue to strengthen teamwork so that they don't perform as badly as they did against Atalanta and being able to get the Bundesliga title is one alternative to increase the self confidence of Leverkusen players or getting a title in the DFB competition is also one way to improve Leverkusen players mentality to get used to facing stronger teams.
They already have sufficient capital for what they can achieve this season. Just imagine that it is a season without defeat in the Bundesliga, they only experienced one defeat in all the matches they played this season, that is a statistic that can make them more confident in being able to play well next season.
It is true that the Champions League will not be easy for them, but keep in mind that they have a coach who has good experience in the Champions League even though he got that experience when he was still a player. However, the experience he has can be passed on to his foster children, one of which is maintaining their mentality when they are under very high pressure. Because it is certain, in the Champions League they will be under much greater pressure.
Xabi Alonso was in the team that I consider to have the best mentality, Real Madrid. He definitely learned a lot there and he can apply that to the players he has.