Non-christians and Christians alike seem to think the existence of multiple stories about a particular person written down after all first parties were dead means a historical Jesus is a certainty. I actually like Christianity and many of the teachings, but for some reason people will accept the worst evidence for this one specific claim.
What's the alternative? This is true of many historical facts we take for granted.
The earliest accounts of Jesus were written only one generation after Jesus's death. Plenty of first-hand accounts were still available. There are many of corroborative accounts of Jesus and other historical accounts of this period.
Christianity had spread to Greece in 50 AD, only 20 years after Jesus's supposed death. In an age where information moved so slowly, it's remarkable that Christianity had spread so quickly.
It's fair to dispute the details of the canonical/apocryphal stories—surely those are contrived—but the idea of Jesus had to come from somewhere. No one else was interested in taking the credit, which should tell us something.
Yes, Mithras, Dionysus, Zarathustra, and other pantheonic cults have quite a bit of similarity to the messianic figure at the center of Christianity as well as its early rites and rituals. Whether Greek, Iranian, Phoenician/Hebraic fusion doesn't really change the convergence that arose as a consequence of the Roman policy on toleration of local cultures.
Mithrasism was a good candidate to replace early Christianity, as they were rival religions. Christianity persecuted Mithrasism starting in the fourth century CE, all but wiping it out.
Information would move quite fast within the Roman empire, especially in the Mediterranean basin. There was a neverending circular movement of goods and people there. Soldiers, mariners, businesspeople, bureaucrats, learned men. Getting from Roman Palestine to Greece by ship was probably faster than getting from Roman Palestine to interior Syria by caravan.
The alternative is that there's no historical character, merely a mythical one. Many of the early stories make much more sense if you adopt that position.
The earliest allegorical story that mentions the character was written around 70 CE - so a good 40 years after the supposed crucifixion.
You assert plenty of first hand accounts were available (they weren't), and that there are many corroborative accounts now (there aren't).
The movement seemed to be spreading remarkably slowly until it was leveraged by an opportunistic emperor in the early 4th century. At that time there were dozens of various sects, the Nicene was just another variant, but the orthodoxy that came out of that adoption meant we lost many of the works and details of the others. That rewriting of history would also address your last point, too.
What present-day meme will reach a similar level of distribution and hazy origins? Will we see future generations speculating about the true nature of Harry Potter, or the Jedi, or Chuck Norris's superpowers, or the one true Nyancat?
I mean, we still have records of Josephus Flavius' (not a Christian and not associated with the early church) writings. He was writing something like 50 to 60 years after the date of Jesus death. However, it should be pointed out he mentioned something like 20 people all called Jesus (in Koine Greek), because it was at the time a relative common soubriquet. No early writers seem to doubt the existence of such a person although they definitely differ on the details.
In short, it's highly unlikely Jesus is purely fictional, and there was almost certainly someone known as Jesus executed by Pontius Pilate. Is this going to make a difference to your beliefs in any direction? No.
Those writings are doing a lot of heavy lifting, since that's about all apologists can hold up as 'contemporary corroboration'.
Antiquities was written in the last decade of the first century, so at the further end of your 50-60y range. As you note, the wording in the Jamesian reference is not very compelling in itself, and historicists generally argue that Josephus would have explained his terminology much more carefully than we see in that section, so there's some reason to doubt it was in the original.
The longer section from that work that apologists refer to is clearly an interpolation. The style is completely wrong for both the author and the context. The previous pages are describing taxes, protests, massacres, etc and then there's this very brief and spectacularly flowery prose that attests to Jesus being the Christ, and to the ten thousand wonders he did, and then on the next page the author continues with 'And another terrible thing that happened to the Jews ...' -- which only makes sense once you remove the clearly forged / added page.
At which point we're really short - a rounding error away from zero - on any corroborating stories from the 1st century.
Independently from being an atheist I’m very dubious about the historic validity of Jesus. Clearly all the supernatural aspects (virgin birth, miracles, & cetera) are to be eliminated as implausible, but even what remains doesn’t strike me as substantiated by historical veracity. Accounts are inconstant and do not gel well with certainties we have about the period (such as Roman censuses https://en.m.wikipedia.org/wiki/Census_of_Quirinius).
We have very little first hand evidence about almost any historical figure every existing, for oldest surviving written source about the Second Punic war and Hannibal's invasion of Italy was written at least 50 years after it happened. The oldest surviving book about Alexander the Great was written several hundred's of years after his death.
