They spent the whole season without their centre-forward and reiterated that they were a great team, first winning the La Liga championship and then the Champions League. There was only one cup missing, I think, with the Mbappe transfer, they became candidates again for all the trophies that can be won next year.
Xavi has left the Barcelona side and it should only be a matter of time before the team announces new transfers. Yes, first of all, Barcelona needs new players to organize its fragile defensive line.
You say they don't have a striker, but you forget the Bellingham factor, who scored in every game at the start of the league and brought them wins. In those games Bellingham always played like a striker and went inside the penalty area. Later, when Vini Jr. returned, he made a significant goal contribution. Joselu, who nobody liked, who was seen as a backup striker, worked wonders. Rodrygo and Diaz scored goals. Real had no goal problems. They had a very strong offense even without Benzema. They will be the favorites next season too, because it's still unclear when Barca will overcome the economic crisis they are going through.
But you yourself brought up the important part in this post: he scored in every game at the start of the season. But now he hasn't scored in a very long time in the league and by far not anything near the number of goals he scored at the beginning of the season. Hence, the Bellingham factor as you call it is a nice to have, but what if it doesn't take effect? Then it is indeed true to say that they lack a pure striker apart from Joselu.
And as much as I like Bellingham as a player, his qualities are everywhere but he can't be expected to score 25 - 30 goals per season. That is why they need someone like Mbappe. That would solve a problem they have. Hard to say so as they won the Champions League now, but it would male them even better.