Exhaustion can surely affect any players mentally or physically that is why this holiday break can sure become a good relief in easing up tension from their previous games, and surely this will benefit not only Dortmund but the other clubs aswell, or Dortmund is still not recovering from the Bayern lost, because of the call of the referee, they are sure to close by one goal in winning from that match, and because of that call it all change, this is just my speculation because of their match against Bayern, Dortmund is not the same anymore, or should I say Erling Haaland is pretty much not the same anymore, some games could surely have an effect to players,
Exhaustion is bad for players because their performance can be worse than normal and worst it might cause serious injuries and harm their career as well.
A break with holidays is good for players to recharge energy and avoid exhaustion but it contains another risk. Players have to be very professional to control what they eat, drink, how they enjoy their holidays but still maintain physical and training activities. If they can control such things, they will return with bad fitness after holidays. We can name some players, Higuain after he had a transfer from Napoli to Juventus, Neymar after latest Coppa America, Hazard after a transfer from Chelsea to Real Madrid, etc.
Haaland might leave Dortmund in a winter transfer. I don't think he will stay in Dortmund after the summer transfer season but in winter one, yes I would be surprised if he decides to leave a bit earlier.