That's right, Bayern Munich will try to prevent the points gap from widening with Leverkusen, besides they will also try to maintain their prestige as rulers of the Bundesliga. Even though he is no longer in the race for trophies at the moment, Tuchel will try not to increase his team number of defeats, so that it will have a bad effect on their status in the Champions League later. In the first half of the season, Dortmund lost at home, so it was not an easy job to get points at the Allianz Arena.
I think Dortmund is also very aware of that now even though Dortmund will also try to get points at the Allianz Arena this weekend. However, because of what you said, it also makes sense that Bayern Munich itself will also try not to lose any more matches with any team in the Bundesliga in order to keep the points gap the same as now. Or at least be able to cut it a little when Leverkusen suffers a draw through their own matches in the remainder of this season in the Bundesliga.
If Dortmund improve their defence, they will play for the championship both in the league and in Europe, but no matter how many transfers they make every year, the same lousy defence does not change every year, they cannot show the success they show in attacking players. Nothing changed in Bayern also. I don't know if Tuchel is trying to make Bayern an England team or not. Maybe there were some dribble passing when Roben and Ribery were there, but I have never seen such a dribble passing-based offence in Bayern. That's why they already have problems against closing teams that press, and their defence is not as fast and skilful as it used to be and they keep conceding goals.