It would be interesting to know what specific tasks are assigned to Flick, but this is classified information (at least for now). Barcelona cannot publicly admit that they have no goal of winning La Liga (due to their status) so now we can only guess what they want from him. If we draw an analogy with Xavi, last season they expected any two trophies from him (even the smallest one like the Spanish Cup). It seems to me that the target level for him is set as “no lower than second place in La Liga and no less than the quarterfinals in the Champions League + some title.”
La is not for some random mediocre team. Xavi Hernandez was relived from his duties because the board simply want what's best and they've seen a competent coach which they probably solve the problem. Barcelona's newly appointed coach, Hansi Flick is currently in charge we should expect the very best from him and Barcelona next season.
We all know the main problem with Barcelona is not a coach; Barcelona's financial problems are the reason why Xavi couldn't function as well as expected. So I disagree with you that Xavi was relieved of his duty as Barcelona's head coach because he was incompetent. Barcelona played with most of their young players this season, which didn't give Xavi a choice but to work with them because Barcelona was not financially stable enough to buy good players. With the addition of Mbappe to the Real Madrid squad, Barcelona are likely not to win the La Liga title because they don't have a squad that can rival Real Madrid for the La Liga title when the new season starts.