I think you can identify Hermeticism with Plato only in the widest sense, about the same as Christianity. Your best source is still _The Egyptian Hermes_ for the historical origin of hermeticism. The argument of that book is sound, rooting hermeticism in greek speaking Egyptian intellectuals and by that route to the philsophy of late empire Egyptian religion (but with much influence from Greek Philosophy, and yes, Plato). It dies out in late antiquity for the most part, though some medieval thought, especially Alchemy, maintains the current. During the Renaissance, with the translation of Plato and the Corpus Hermeticum it gains a strong resurgance, but it is in a highly transmuted form primarily through textual, rather then oral, transmission. But the strength of that transmission is, I think, validated by the ease with which it quickly come to resonate with other forms of medieval thought, such as Kabballah and Alchemy, that are also heavily influenced by the earlier hermetic thought (which may best be identified, widely as 2nd to 6th century CE (or AD or EV as you may prefer).

So here is my timeline

2nd - 6th century CE Early Hermeticism (primarily growing out of Egypt, and a sister to Gnosticism, Christianity, and non-rabbinic Judaism)

6th - 14th century CE mostly preserved in other modes of thought--Alchemy, Jewish Mysticism, Islamic Philosophy and Science

14th - 16th Hermetic Renaissance, primarily through textual transmission but also through the above mentioned modes of thought

17th+ Modern Hermeticism, primarily leaving elite philosophy and being transmitted in popular literacy with printing, and in the development of esoteric brotherhoods

That's a rough and ready timeline and I am not consulting any sources, just doing it off the top of my head based on memory.

I think the ideas that hermeticism comes from a mycenean greek origin or the like is a bit of stretch--not impossible, but highly speculative. I think a lot of really comes from the alchemical mixture of a lot of sources that grew out of the interpolination of culture provided by hellenistic greek and roman conquest; to speak of a "hermeticism" before late antiquity is foolish.