Many thought that Barcelona would play their second tier squad in this match. But Hansi Flick did not want to take any risk in this match. Because Barcelona rested Yamal and Raphinha in the match against Osasuna. And it was in this match that Barcelona was defeated. Perhaps this is why Hansi Flick did not want to take any risks in this match. So Barcelona played with their main squad. And managed to score 3 goals in the first half.
Hansi Flick doesn't want to underestimate the opponent. Sure, he is aware that Barcelona position isn't safe yet. There are still many remaining matches. Although the point gap with Real Madrid is getting larger, Hansi Flick wants to secure Barcelona position in the top of the table. So, Hansi Flick must target to get full points in each match. Unless Barcelona main players have injured problems, I think Hansi Flick must always use the first tier lineup.
Yamal, Raphinha, and Lewandowski have no problem with their fitness. I think it is no problem to let them playing in the match. However, I'm a bit surprised that Yamal and Lewandowski weren't replaced with other players in second half of the match. Barcelona have scored 3 goals in the first half of the match, I think it will be more wise to let them taking a rest in second half. Well, not sure why it happens, it may be a part of Hansi Flick tactics.
You are right that Hansi Flick did not underestimate the opponent even when the opponent was weak. I would say Hansi Flick has learned from previous mistakes. If Barcelona's starting XI against Osasuna had Yamal and Raphinha, Barcelona might have been unbeaten in La Liga by now. These two players are now two of the most important players in Barcelona's squad. It is very important to have these two players in the squad even if they cannot score. So in this match we saw Yamal, Olmo and Raphinha in the squad.
Barcelona's performance was a bit slow in the second half. I believe that Barcelona would have been able to win this match by more goals if they had given their full effort. But after taking a 3-0 lead, Barcelona played a bit slower in the second half. Even then I think Lewa, Yamal and Raphinha should have been substituted in the second half. Because if these players face injuries, Barcelona will also face a big loss.