Spin: Pliny the Younger and Josephus talk about the Christians. According to my edition of The Jewish War, the fragment you talk about was discovered in 1960. Moreover, Josephus spends two pages (p. 138-39) on Pontius. Suetonius wrote, "Punishments were also inflicted on the Christians, a sect professing a new and mischievous religious belief..." (p.221, The Twelve Caesars). As you note, we also have Philo (a reference I haven't read) and Tacitus, plus the Gospels. There are also the writings that didn't make it into the NT, such as the Gnostics that attest to the existence of Jesus. For God's sake, how many sources does one need, remembering that we are talking about an event that took place about 1,982 years ago?
As for archaeological evidence, why should there be such evidence? Jesus didn't build a civilization, didn't write books. He preached a new version of Judaism. How could one expect relics? There were no ways of recording his speech, except through writing, and the fact that he was itinerant would have mitigated against a scribe recording what he said, particularly since he spent a good part of his time fleeing the religious authorities.
What group of historians doubts the historical Jesus? As far as I know, none. Spin, I consider this claim of yours an aberration in an otherwise excellent record of scholarship.