Barcelona is delaying this deal, they are supposed to be done by this because the player get more hype after the just concluded EURO 2024. Many teams are targeting the player including Arsenal, but I think the player is interested in joining Barcelona especially as their partnership with Lamin Yamal in the EURO 2024 gives a better result.
If Barcelona is able to sign Niko Williams, it will be easier for the team and they will not be less competitive as we may think and they can challenge for the Laliga title alongside Real Madrid because these young players can do everything to win match as we have seen in the Euro 2024.
You know Barcelona is still in the financial battle and are not done with that yet but nevertheless, they may not allow that get in their way to building a good squad for next season, so far Barcelona have been making really interesting moves, though that with Niko Williams is inconclusive yet but they are very keen about it and it's very likely it will pull through and like you mentioned, it's going to be a sleek combo with yamal and the fact that the guy loves Barcelona already will get him to putting in only but his best and nothing short of it.