But then he goes on to say that the highest good is prudence. It seems that Epicurus said this, so he's on solid ground there.
Right. The highest *instrumental* good thing.
Nevertheless, I think it is useless and counter-productive to even discuss a "highest good" once you have established that pleasure is not it.
I don't think we've established that.
And providing the most pleasure is the only criteria we have to consider a good "highest", right?
No. We're getting hung up on the word "highest" here. It's not a value like "best." Well, in a way but not primarily. It's the highest at the summit towards which all other good things point. Or, within this discussion, to which all other good things serve as instrumental things.
And even if you could find one "highest good", so what? Are there some important goods that almost everyone will want to have? Sure. And Epicurean ethics should talk about those. But there is no need for a universal ranking to apply to everyone in all circumstances.
It's not a ranking. We're not putting good things into a hierarchy. I'll get to Epicurus's prudence in a moment. I've had this discussion with Cassius before, too. Pleasure is the only good thing that doesn't point to something else. That's what makes it the "highest", telos, goal, summum bonum, etc. The cheese stands alone, to use the Old McDonald had a Farm song. Practical wisdom/prudence can be characterized as the most cardinal, chief, greatest good thing (not The Good) because it is through this virtue that we're able to make them best decisions for what pleasures to choose and from which pleasures to flee. Butt it's still subordinate to and instrumental for achieving pleasure.
And so the foundation of all these and the greatest good (τὸ μέγιστον ἀγαθὸν) is φρόνησις, practical wisdom. On this account, practical wisdom is prized more dearly than philosophy itself, and from practical wisdom springs forth all the remaining virtues, teaching us that a pleasurable life does not exist without the traits of wisdom, morality, and justice; nor do the traits of wisdom, morality, and justice without pleasure: because the virtues grow together with a pleasurable life and the pleasurable life is inseparable from these.