But the whole point of Psychohistory (as it’s presented in the books) is that there aren’t any “Chosen Ones” at all. Sure Salvor isn’t one, but Gaal isn’t either; no one is. That’s the entire point.

Under Psychohistory, it shouldn’t matter who the leader is; what matters is the forces on the population as a whole and that populations respond to those forces in predictable ways. Individual leaders come and go and really don’t matter very much on the big-scale stuff that Psycho-History is making predictions about.

It’s the whole basis of the first several books that this “large forces acting across whole populations” model is the ‘true’ way that civilisation-scale history works and why the future history is so predictable (before stuff starts going a bit off the rails later on in ways that I’m not going to spoil).

To me, if you’re not using that concept as the bedrock of the story you’re telling, then you’re not telling a Foundation story. (It might be a quite good science fiction story of course, just not Foundation)

I'll grant you, Salvor is definitely turning into a false Chosen One (also, this would be the first Seldon Crisis which they weren't even supposed to know of until after the first one, and she's already name dropped the term). She's got an idiot ball grafted onto her somewhere that she just can't shed.

However, Seldon, to Raych, tells him that his decision with respect to Gaal will determine the fate of the galaxy. That's way too much weight for Seldon to place on one person in the context of the series, especially at this point unless he already knows about people like The Mule (which would be unfortunate, that'd be something best explored as it throws a wrench in your plans, not from the beginning of the series).

All of those themes are in the books, it's why he sets the Foundation up to make an encyclopedia. They aren't supposed to know, because (too much) knowledge of the future leads to potentially massive change (a variation on the Heisenberg Uncertainty Principle). So they can't know their real purpose (or one of their purposes) without changing the results.

And the series subverts that from the start, why? What does it do to make it better?

That last question is the key for me. A show or movie is never the same as the book (or very rarely). There are a lot of things that can or should be changed for adaptation. But why would you subvert one of the key themes that defined the book series?