There is still a month for the beginning of the new season. Good enough time to complete signings and get ready. I wonder how good Dortmund will be able to start. The players will have less time to rest because of the Club World Cup.
I doubt Erik ten Hag will be successful with Bayern Leverkusen after defeating in the pre-season match against Flamengo-U20 over 5-1, indeed as pre season match but the opponent teams as dominance by youth players how easily Leverkusen conceded until five goals.
It is too harsh to make an assumption about a whole season only depending on a friendly result. I don't agree with your opinion. Their lineup wasn't even full of main players. Like I said before, they had U19 players all over there.
When it comes to signings this summer, they are changing. A new system and new players. They lost some important names. But hopefully Erik ten Hag will manage to build a team of his preferences.