I think the problem is your use of the word "subjective". Two astrophysicists may reach different conclusions as to the age of the universe, but that doesn't mean their opinions are subjective. They may be using different data, they may have started with different premises, one or both of them may be wrong, etc. None of that means they are giving subjective opinions.
The same is true for probable cause (and "beyond a reasonable doubt"). There are objective tests for both, but even rational humans may reach different conclusions without resorting to subjective assumptions.
But the burden of proof does shift with an affirmative defense; that's why such a defense is called "affirmative". I was a juror on a self-defense murder case in CA. It was made quite clear to us that the defendant had to prove he acted in reasonable self-defense; it was not the prosecution's burden to prove he had not. (Of course that didn't prevent the DA from presenting contrary evidence; it just wasn't his or her burden to disprove the claim.)
According to its author (whom I heard on TV), the Florida SYG law was intended to relieve self-defense killers from spending years and their life savings on the legal process. Sounds nice in theory, but we see in practice that LE is left with the testimony of the survivor, while the dead man "tells no tales".
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