I disagree with you, Bayer Leverkusen was weak this season because Bayern Munich was no the only team they faced this season that made them end up not getting any trophy. Bayern Munich got better and wasn't in poor form like last season and Leverkusen became weak as they couldn't win matches that was expected of them to win.
Xabi Alonso is out of the team with Ten Hag brought in as his replacement untill the team is in better shape I can not rate Ten Hag well because he failed as a Manchester United coach.
Differentially it has been observed that Bayer Leverkusen has been much weaker this season compared to last season because they were champions the previous season. Their statistics clearly show that they have won 9 fewer matches this season than last. Maybe they are going to make big changes in this regard now but I would say that their biggest weakness at this stage would be the departure of Xabi Alonso because the management of this team couldn't do anything important to keep him.
It is also a matter of concern now that their midfielder Jonathan Tah has left the team, so what might be Bayer Leverkusen's attitude after this? I think Bayer Leverkusen has to make some expected contracts such as adding Garnacho and Antonio, maybe it will be very helpful for them.