but if the underlying assumption is that short-term physical pleasures are equally as important as long-term mental pleasures
Yes that is the tricky part and I do not think that Epicurus would say that that can be presumed. Most generally, I think he would say that that decision has to be made individually by each person and according to life circumstances, and that it will prove to be applicable to some people but not to others (so therefore not a "general" rule). Plus, to be clear, I am not limiting the circumstances to "material well-being." People rank their pleasures in radically different ways, and two children born in the same house to a life of luxury, or to a life of poverty, could easily reach different conclusions on how they choose to spend their time to generate the most pleasure for themselves.
But I think it's pretty clear that Epicurus did not reach such a conclusion in his own case (he devoted his life to philosophical study and indeed controversy).
So the very first piece of evidence in unraveling Epicurean doctrine (how Epicurus lived his own life) would not be consistent with applying that premise across-the-board to everyone.