Dortmund concede a goal very easily. You won't be successful if you score goals that easily. In modern football, it is very important to maintain the defensive-offensive balance. The goalkeeper will keep the team alive with the saves he made in a game they played very badly. Whether the defenders have air control or ground play, they must be above a certain level of quality. 6 and 8 are the key positions in modern football. In front of the defence, there must be a player who will play both sides of the game in front of the centre-backs, who will play the ball forward by making good passes, who will be a good central midfielder in terms of his feet and physically. I think Dortmund lacks these aspect of the game, even though of course I believe they are a great team.
Erling halland remains their biggest fortune for now and from the look of things, he's improving everyday that passes by and that's all to the benefit of the club so I don't really thing they have a problem with thier strikers and wingers too. The problem is with the defenders as you said and I think they need the likes of ; Declan rice, Marcelo broroviç,fernandinio and the rest of the strong defensive midfielders. If they have a good validate on standby for substitutions, then they can have a different gameplay entirely