On paper Real Madrid are far superior to Salzburg and Brest, but anything can happen as long as the match is still in progress. Although the chances for both clubs are very slim, it could be very decisive and could punish Real Madrid. What just happened at Anfield when Man United held Liverpool to a draw could happen to Real Madrid.
This match is very important for Real Madrid to determine their fate in the UCL, they must take advantage of every opportunity and prevent the slightest mistake so as not to lose points at a crucial time.
Honestly I will not underestimate RB Salzburg and Brest, moreover Madrid is still a bit unstable when playing against teams below them. However, because this is the Champions League, at least Ancelotti and his troops understand the atmosphere on the field much better. Talking about the Champions League, the most important thing is the mentality to play until the end, therefore RB Salzburg and Brest must be able to match the mentality of Real Madrid game. That is the only key that other clubs need to prepare when playing against Madrid.
Madrid we know as the favorite team so far when playing in the UCL, but if we look at their performance this season, we have to admit that they are not in perfect condition like the previous seasons, so although Salzburg and Brest are not teams on the same level with Madrid, but we cannot underestimate them, especially in the last match of the league phase, In my opinion, the pressure and burden will actually be on Madrid side later in the match, because indeed their position is still difficult to qualify for the next round.
Currently Salzburg and Brest need to focus on mental and physical preparation to be able to overcome the pressure from Madrid like you say, in addition, the right strategy and the ability to stay calm under pressure will be very important when facing a big team like Madrid.
I am also sure that Madrid will be able to win the remaining two matches, but I think the victory will not be easy for them to get.