Watching 300 was your first mistake there...
Yes, it's stylised; impressionistic, even. Metaphorical. Similistic. Art rather than substance.
Excuses made and grains of salt accounted for, it is also anti- history of the most despicable sort, deliberately intended to obscure, delude, and hide the actual truth of the past.
Start with Thucydides,
The Peloponnesian War- easily Gutenbergable,
https://www.gutenberg.org/ebooks/7142, for a more general take and specifically the turning point when something like that, a surviving and thriving Sparta, was closest to being within reach.
The problem is that the prime candidate for your reformer is probably Lysander, who was largely a self made man, which cuts both ways- on one hand ego and determination, on the other contempt for those who didn't claw their way up as he had- had strong ties to Persia, was notoriously personally vindictive and according to some commentators at the time- Nepos, Duris, Argesilaus- enough of an egomaniac to attempt to have himself deified, possibly successfully.
If you look ahead another moment, Athens makes a comeback in the 370s, a generation later, because the Spartans make for intolerable overlords, even worse than the Athenians. The Athenians regain their independence, the Thebans defeat the Spartans
on land,- Leuctra- and the lower Greek powers are basically in a state of cold war/ winding up for another go when Macedon intervenes and squishes the Athenians, largely suppresses the Spartans.
Subsequently, hm, there's something definitely flawed in the Spartan character, something of the beta male about their relations with Persia, Macedon and later Rome.
Leonidas was the exception that proves the rule; he stood, on a point of principle, for Greek freedom, to the death- and very few of his predecessors and descendants chose the same. They accepted Persian help against the rest of Greece, they growled at Alexander but did nothing, they were allies and later submitted to Rome.
That glorious exception aside, they were seldom if ever willing to fight for a cause and against the odds. In the face of Persian divide and conquer, they were all too willing to be divided from the rest of Greece. Power and pragmatism, and very little culture and civilisation behind it.
I can't think of an obvious point where they could have changed; most of the historical turning points that could have made them different would have destroyed them, if anyone would break but not bend it was them.
Although willing to side with Persia, they never assimilated Persian culture to any noticeable degree, the exchange of ideas (as opposed to money) was almost nil; they avoided the great Hellenic surge eastwards- perhaps that might work. Have Alexander of Macedon make them an offer they can't refuse, have them dragged into that. Perhaps they might learn. (Bit late, though.)