Sebastian Haller is a really good signing for Dortmund now. Maybe it is very difficult for him to replace Erling Haaland but Dortmund wouldn't be in such expectation either. They must have just wanted to sign a trustworthy striker to be effective in attacking as always. They have made some moves for their defense too but I hope it can be enough for them. Because it is their weakest point to improve right now. Their weakness in their defense caused them to be out of the title race early.
It will be difficult for Dortmund to find a very energetic player like Erling Haaland, Although Sebastien Haller did very well at Ajax last season, I don't think he will be able to replace Haaland at Dortmund. But, back to your statement that it looks like Dortmund are not looking for a replacement for Haaland, they signed Haller only as a starting line-up for the main players. Overall, I also hope Haller can make a good performance with Dortmund.