It strikes me as odd that several posters use the lack of contemporary documentation about a non-miraculous Jesus as evidence for the position that there never was one, yet are much less stringent when it comes to their side.
Then all of a sudden, conjecture about mystery cults, secret plots to erase the 'real' history of a religion, and inference that 1st century writers were 'really' referring to a mythical ancestor, are deemed admissible.
Lack of contemporary documentation is only one of very many problems with the Jesus stories. And it's probably been the least mentioned of all the problems raised in this thread. But the reason that lack of contemporary documentation is mentioned as a problem, is that in any historical study one of the first things that historians would like to find is contemporary evidence ... if they do not find that, then the case is significantly weakened. So that's the entirely general reason why it's often pointed out that there is no such contemporary evidence of Jesus.
Then as far as your subsequent comment about sceptics relying upon quote
"conjecture about mystery cults, secret plots to erase the 'real' history of a religion, and inference that 1st century writers were 'really' referring to a mythical ancestor" ...2 and saying those things are
"deemed admissible", I don't think that sceptics here have argued that as the main problem with the Jesus stories at all.
What sceptics have argued as the main evidence against a real Jesus are factual things such as the following -
1. Most of the stories about Jesus involved miracles which were believed literally true until as late as about 1850. But since then, modern science has shown that all those miracle claims are almost certainly untrue.
2. None of extant biblical writing is anywhere near as "contemporary" as the church had tried to claim for most of the past 2000 years. It is not for example within a few decades of the supposed death of Jesus. The first more-or-less properly readable copies providing the detail of what was said about Jesus are P46 circa 200 AD, which actually says nothing about it's author or anyone else knowing a human Jesus, and then 4th to 6th century and later from the four canonical gospels.
3. None of the people who wrote any letters or gospels describing Jesus, had ever met or known any such person as Jesus. He was just a prophesised messianic scion of God who was believed in by those various anonymous writers as a matter of their fanatical 1st century religious faith.
4. Similarly, for most of the past 2000 years the church had always claimed that Josephus and Tacitus provided independent evidence of Jesus that was written within about 70 years of the death of Jesus. But that again has turned out to be completely untrue, and in fact the first extant copies of any such writing were apparently produced not circa.100 AD, but c.1000 AD!
5. The credibility of all that biblical and non-biblical writing has been seriously undermined by the discovery that the Christian copyists who actually wrote all of the extant texts (there are no originals), were in the frequent habit of altering any passages about Jesus whenever they thought the passage should say something different to what was originally written.
6. As authors like Randel Helms have shown, the writers of all 4 gospels were certainly using the OT as a source for their Jesus stories.
7. There is no genuinely independent writing about Jesus, except for the bible. E.g., there are no independent Roman official records naming Jesus in any court proceedings or any census or any records like that. And there is no physical evidence of any kind either ... although Christians and the church have tried very many times to produce fake physical evidence such as the Turin Shroud, the Bone Box of James, and various bits of the wooden cross and it’s old nails etc.
8. The biblical stories of Jesus have clear similarities to the fictional stories of other dying-&-rising gods that were believed from an earlier time in that same region.
9. Although the Jesus story is said to have taken place in Judea, all the extant biblical writing has been found in Egypt.
Those are the sort of reasons presented by most sceptics. Not just sceptical claims of saying that none of the writing is actually contemporary with the supposed lifetime of Jesus, or that there is actually nothing written by Jesus himself, or even anything written by anyone who ever claimed to have met Jesus ... but rather, the above sort of reasons (1 to 9) are the sort problems pointed out most often by sceptics.