The purpose of crucification is striking fear on slaves so they would be more obedient. Thus, no merciful acts would ever be commited in such an event. The more and the longer one suffered on the cross, the better the state feels about it.
That said, I agree with CH on the technical parts.
From what I have read on this topic, I should say the same things as Cloth Hat has already said. Yes, to have legs broken is very painful, but it does ensure a much swifter death than a slow one over the course of several days with the unpleasantries of blood loss, festering wounds and perhaps even nibbles from vermin.
And yes, crucifixion was intended to be a very humiliating type of agonising death.
Hades, just imagine. What’s a more painful death, dying of like, blood loss and gangrene and rat bites and thirst/hunger, or dying of broken legs and asphyxiation? I mean, I don’t know the answer - these aren’t things I think about a lot usually But I suppose it depends on the person being crucified, what’s a more painful way for them to die.