I might disagree about exactly how much ataraxia is under our control--it's quite unclear, I'd say. Clearly there are some sources saying we can have it under many different circumstances; but PD 4 seems to imply that fatal pains might be pretty rough for a short time, and there's the evidence in DL 10 that not all circumstances can lead to a good life.
Ah, this might be a good way to get at why or how we might have a different perspective on this. My understanding of our control over our own ataraxia stems from my view of tranquility/ataraxia being (possibly) less expansive than you.
I maintain that ataraxia is best understood specifically as the state of mind engendered by the eradication of false/empty beliefs and the fears and anxiety they produce. Once those beliefs are totally eradicated from our minds, pulled up by the roots, they don't come back and the tranquility - literally the undisturbed state - is solid. The eradication of those fears and anxieties is final IF we've really understood and internalized at the gut level, asleep and awake, the understanding that the gods present no reason for fear, that death is nothing to us, that pleasure is the goal and should be the North Star by which we steer our little boat. Pleasure writ large as Epicurus defined it, which means we undergo some pain if we know there's pleasure on the other side of the pain.
If I remember, you include ALL fears and anxieties within ataraxia; which MAY be the case if it is understood that there is no reason for fear and anxiety IF we steer our boat by the goal of nature, which is pleasure. My understanding is that if one defines ataraxia/tranquility to be encompassing of every fear and anxiety, it has to be understood in that way.
I go back to Metrodorus, Fragment 5: "Metrodorus, in his book On the Source of Happiness in Ourselves being greater than that which arises from Objects, says: 'What else is the good of the soul/mind/psykhe but the sound state of the flesh (τὸ σαρκὸς εὐσταθὲς κατάστημα), and the sure hope of its continuance?'" I take this to mean the entire body - body & mind - taken as a whole, the Good of the Mind is inextricably linked with the stable state of the body
PD4 does say that fatal pain is bad, but fatal physical pain is short (per Epicurus' writings) and we know from his example that eudaimonia is still possible under pain.
I'm curious to explore what you refer to when you mention "not all circumstances can lead to a good life." I certainly don't disagree, especially if you're referring to chasing after empty desires. We're not guaranteed a good life if we persist in empty desires, and we're not guaranteed a completely pain free life if we're mortal.
PS: I woke up with other ideas on how to explain my perspective on how much control we have over our own feeling of ataraxia. Martin Ferguson Smith's Oenoanda inscription is instructive in this (Formerly fr. 34 VI-VII, now separate fragment 38 I-II):
Let us now [investigate] how life is to be made pleasant for us both in states (κατασ̣τήμασι) and in actions (πράξεσιν); and let us first discuss states, keeping an eye on the point that, when the emotions that disturb the soul are removed (τῶν ὀχλούντων τὴν ψυχὴν παθῶν ὑπεξαιρεθέντων), those that produce pleasure enter into it to take their place. Well, what are the disturbing emotions? (II) [They are] fears – of the gods, of death, and of [pains] – and, besides [these], desires that [outrun] the limits fixed by nature. These are the roots of all evils (αἱ ῥί[ζαι] κακῶν π̣ά̣ντων), and, unless we cut them off, [a multitude] of evils will grow [upon] us.
So, we need to cut those fears of gods, death, etc. at the root so they don't grow back. I find it interesting that VS79 has a negative related form of the word Diogenes uses here for troubles AND a word related to ataraxia for peace: VS79 He who is as peace within himself also causes no trouble for others. ὁ ἀτάραχος ἑαυτῷ καὶ ἑτέρῳ ἀόχλητος. The "within himself" (ἑαυτῷ) also reminds me of the title of Metrodorus' book.