Would anyone know where I could find the works in print of such Fathers as the Cappadocians, Maximus the Confessor, Gregory Palamas, John Damascene or Symeon the New Theologian in the original Greek, with or without translation? I don't believe these authors are published in the Loeb collection. Are there other editions besides Migne?
Besides Migne, you can find them in the Sources Chretiennes series, where the original text is published side by side with a translation in French. From those that you mention, perhaps Palamas is not published in SC.
Be advised that the Patrologia Graeca is in about 166 volumes, several of these are freely available here. It is all perfectly legal, the books are from around 1860, so the copy-right is long expired. I don't know if you will find the authors you are looking for, but, in my own view, it is worth checking. It is a pity to pay for new books when there are available old ones just as good.
For those who are looking for an (almost) exhaustive database of complete texts including Migne Patrologia in the original languages from both Greek and Latin Church Fathers, the following should be very much worth exploring - Greek texts - Latin texts
Besides theological texts, the database includes works by pre-christian authors such as Plato, Aristotle, etc. , and other texts of the Byzantine era, such as imperial edicts, church council documents and more.