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 against Salzburg, maybe this will give their players more confidence.
Honestly, Real Madrid had some issues at the start of the season, with some injuries as well as team chemistry issues and Mbappe adjusting period, they weren't that great. But looking at it now, I can say that they are doing mighty well and I do not think that it is going to matter that much ,we could make this work one way or another and they could win this game.
I actually believe they will win both games, and easily be in the playoff stage, they are not going to be in top eight that is guaranteed already, but they will end up in playoff stage and I am not so sure if there are any teams which could do that much better than Real Madrid when it comes to playoff stage as well, feels like it will be a little different. So, they will be fine.