They lost two best players in two seasons, Sancho to Manchester United and Haaland to Manchester City. Marco Reus got an injury and missed most of first part of season. Only Jude Bellingham is not enough for Dortmund. Moukoko is still a very young player and he needs more time.
It causes problems to Dortmund and affects their performance in Bundesliga. Their problems expand to their defense too with an old defender Hummels. This season can be a failed one for Dortmund because of their problems which are supported by sudden good performances of other clubs. Dortmund will have to play with their best to get a fourth position in Bundesliga at least.
Any position lower than the fourth position will be terrible for Dortmund. They will be separated into pieces and star players including Jude will leave.
It's no wonder that Dortmund are not in good shape this season, but they are a club that can produce football stars, some of their star players after going to other clubs quickly become quite productive players like Haaland. well this season Dortmund may need a transition in returning to their best form and I'm sure they will close this season in the top 4. Week 16 of the Bundesliga hasn't started yet and there's still time for each team to make good changes in their game before they face their opponents again in Week 16.
Things are a bit more difficult for Schalke, of course. They are at the bottom of the league and will have to work hard to get out of this position. They need to make good player transfers. Staying in the Bundesliga is not easy because many teams are very strong. Schalke, Bochum and Stuttgart are the teams that are in the relegation pot. They must have spent this break mostly getting ready.
Thomas Reis should be able to get Schalke back to winning ways once the league resumes. As you said, Schalke has moved quite quickly in the transfer window where Schalke's management has received an injection of energy from Niklas Tauer, who is on loan from Mainz. And maybe several other players have been targeted by Schalke in this transfer window. in the upcoming match Thomas Reis must be able to make a big difference against Eintracht Frankfurt in order to get an early victory after a long holiday. but seeing the quality of these two teams I am pessimistic that Schalke will be able to steal points when they travel to the Deutsche Bank park headquarters.