If we talk about tactics, of course it will be different and the difference will probably seem quite large. I think Hansi Flick is much more reliable than Xavi and everything will change as long as no problems come. Maybe Barcelona should be able to strengthen their players' physical strength so that they don't get injured as often.
I am very sure that if most of Barcelona's players can avoid injury then they will be much calmer in navigating the competition next season and at least they should not be outside the top four in Hansi Flick's debut as coach. However, if we talk about competing with Real Madrid, it will still be quite far away and it will be difficult for Barcelona to match it.
Xavi has proven his capacity and he has given the title of La Liga and the Spanish Super Cup to Barcelona while the team is down with financial problems. I think it is not appropriate to conclude that Flick is better than Xavi. This season Xavi can also be said to have worked quite well, but he did not get full support because of some things that made internal problems even more tapered. Flick is also a great coach but able to prove that he will be better than Xavi and in my opinion all of that will not be easy.
Both Hansi Flick and Xavi are exceptional managers who have proven their abilities by delivering trophies to the clubs they have led. Comparing them solely based on the number of trophies they have won is an unfair assessment. Barcelona is a complex club currently facing a financial crisis, making it challenging for any manager, including Hansi Flick, to bring home trophies on next seasons.
Xavi was very lucky to be able to do this when he was Barcelona manager, Hansi Flick certainly has the opportunity to repeat it, but the challenges ahead will be even more difficult as Madrid looks even tougher. If Hansi Flick is not equipped with an adequate budget, regardless of how Barcelona deals with the crisis, then Barcelona chances of winning a trophy next season are quite low. Unless Madrid suffers a setback, and even that is impossible to happen.
Apart from that, the injury factor always hinders the team development, it is difficult to achieve the target if many important players are injured. This season De Jong did not perform optimally due to injury, in the middle of the season Gavi also had to be sidelined. On the other hand, I think the presence of Hansi Flick as Barcelona manager next season will at least be able to keep the competition fierce, so that Madrid doesn't dominate too much.