Pliny is authentic, but they were not prosecuted for being Christian. Originally, Pliny doesn't even know what Christianity is. Their crime was assembling without an imperial licence, not for any specific religious practice. Pliny's problem with them was political. Now did some of them die, yes. Rome could be a very tough place. But Pliny wasn't after them for their theology. But, Pliny aside, there was nothing systematic about 1st C persecution.
Poison Pill: I find it fascinating that you just state something controversial (Tacitus is a forgery) without citing any evidence whatsoever!
I doubt I’ll convince you, because that would mean a major adjustment in your worldview, but for other readers I’ll offer this:
Even Bart Ehrman, famous New Testament author and NO friend of Christian causes, says this:
“I asked one of the prominent scholars of the Roman world, James Rives, who happens now to teach at UNC. Anyone who wonders about his credentials can look them up on the web; he’s one of the best known experts on Roman religion (and other things Roman) internationally. He has given me permission to cite him by name, as he is willing to stand by what he says.
My initial email question to him was this:
I’m wondering if there is any dispute, today, over the passage in Annals 15 where he mentions Jesus (whether there is any dispute over its authenticity).
His initial reply was this:
I’ve never come across any dispute about the authenticity of Ann. 15.44; as far as I’m aware, it’s always been accepted as genuine, although of course there are plenty of disputes over Tacitus’ precise meaning, the source of his information, and the nature of the historical events that lie behind it.”
https://ehrmanblog.org/fuller-reply-to-richard-carrier/
Likewise, Wikipedia dismisses the forgery theory:
“Suggestions that the whole of Annals may have been a forgery have also been generally rejected by scholars.”