Bayern Munich did not become German champion for many years because they are so good. They often became German champions because the other teams were worse and did not perform consistently well throughout the season. There are 34 matchdays and the players from Bayern Munich have the mentality to take their chances until the last second. Only when the whistle blows after the 34th match day the game is over for them.
I don't think it's mostly about performance, because at some point Dortmund had quality players and has produced top class players but they sold them off to other clubs for cash benefits, they would had been a very tough team and competed with Bayern to win the trophy, Bayern Munich is more successful than them, because they invest alot on quality players and use them to achieve trophies and sell the players when they're incompetent unlike Dortmund that sell players when they're at their peak.
Well so far Leverkusen is improving and with how they're going they may likely be the team that would stop Bayern's dominance in the Bundesliga over the years, though it's not going to be easy winning a team that's dominated for decades, Dortmund tried and almost got it but failed let's see if Leverkusen would.
The point is that each team has a different way of managing its team, as you said about the difference in the performance of Dortmund and Bayern Munich, in my opinion these two teams are reversed in terms of managing their teams, but honestly I don't say they are bad, which is certain as you said Dortmund has a very different way compared to other clubs in managing its squad, which is where when other clubs are improving their club's performance by bringing in a lot of new quality ammunition but Dortmund is the opposite, they are always disciplined with habits that of course have an impact on their performance, selling some of the players they have managed to form perfectly in order to get large amounts of money and also sometimes Dortmund sells quality players to their rivals. Honestly, I don't understand the way they do this, what is certain is that money is the main thing for Dortmund and with that, it is commonplace when Dortmund struggles in the title race at the end of the season because their performance is difficult to be consistent.
True, this season Leverkusen is one of the clubs that will be very threatening for several clubs that are in the hunt for trophies in the top five and especially for Bayern Munich itself, of course Xabi Alonso's squad has a great opportunity to break Bayern Munich's dominance as defending champions.