Real Madrid will have to take this game serious, because they need these 3 points more than we can say, because if Real Madrid didn’t win the two last games, then I don’t think they will be among the teams that will be present in the round of 16 this season, which means they have no hope anymore. Although they have the highest chance of winning the game against Salzburg, it is a home game for Real Madrid, so it will be like a sure win for Real Madrid with no stress. Moreover, as you mentioned, Real Madrid is better than them in terms of strength and many other aspects.
Real Madrid must take this game seriously because the way they have performed so far, it may not be entirely certain that they will win against Salzburg. Moreover, the most important aspect for Real Madrid is that if they lose unexpectedly against Salzburg, they may not be able to go to the next round of the Champions League. Real Madrid is a very strong team in La Liga but Real Madrid is not a strong team in Champions League. They are 20th in the Champions League standings, Real Madrid are currently far behind in the Champions League. Also, you mentioned that Real Madrid will be playing at their home ground, maybe this will give their players more confidence.