It just occurred to me… with a name like “Great Thinker” and our primary source being the Atlantis guy talking him up, how sure are we that Socrates actually existed?

So… we’re pretty sure he existed, because Plato is our main source but not the only one who talks about him.  After his death,  we also have philosophical texts written about him by Xenophon, another of his students.  More importantly, while he was alive and long before Plato started writing philosophy (possibly even before Plato was born), Socrates was parodied by Aristophanes in his comedy, the Clouds, so it’s pretty definite that he wasn’t just completely made up in hindsight by Plato.  There are comments in some of Plato’s dialogues suggesting that he was trying to undo the play’s effect on Socrates’ reputation.  Also, although the events of the dialogues are fictionalised (with the exception of the Apology and maybe parts of the Phaedo), almost all the characters in them are real people, attested in other historical texts like Thucydides’ Peloponnesian War or Xenophon’s Hellenica, whose positions in the philosophical debates reflect their real reputations and life stories.  It would be… weird, put it that way, for Plato to construct such an elaborate fiction of a guy who never existed, tie it into the lives of so many other people, many of whom were Plato’s own friends, acquaintances and relatives (several of them dead by the time Plato started writing, most of them not peacefully), and give him a backstory deeply woven into the traumatic events of the Peloponnesian War.  If nothing else, I suspect it would have been in very bad taste.