I forgot about Donny in the OP when listing the senior midfielders, but I can't imagine anyone of them has both the physical engine and the technical ability to be developed into a role similar to what FDJ excels at. I watched quite a few Forrest matches last season and I like Garner, but I don't think he's that type of dribbler or ball carrier. He has the long range pass, he will keep developing and could make it at United (definitely wouldn't want him sold before we see whether he can make it here) while Zidane Iqbal has already shown that he has the technical ability to resist the press and the range of pass, rough though he still is (of course).
@Ekeke I don't think Hannibal has been done too many favours here in terms of his long-term development, he doesn't really have a set position, has been deployed in various ones while he started out as a deep lying midfielder. He's recently said he'd happily play in whatever position the manager wants and he's confident in his own ability. It can be a problem though, for a player breaking through, if they haven't been given a defined position. Well, I think he lacks the physicality (like Iqbal) but he clearly has the technical ability, the real question with them both is if they have the tactical discipline - and they're well placed training with the senior team under a coach-aholic like ETH.