The fact that Jesus lived has already been proven.
Actually, Josephus in particular has pretty well been shown to have his writings added to by a later revisionist who was desperate to have some historical mention of Christ. You can look up that argument on your own.
There are also many statements in the Bible that have been proven by Archeology. So your argument has no legs to stand on.
Each datum in a collection of information stands alone and either is true or it isn't. Attempting to infer truth of some parts because the others have been show to have some historical basis is an egregious logical fallacy.
If I take a collection of random historical data and intermingle them with some random unprovable data that I've completely made up, and then I publish it, the fact that the facts are shown to have some accuracy makes the stuff I've completely made up true?
Of course not. But neither does the the truth or partial truth or historical accuracy of one claim (the existence of a certain city) prove anything else other than the historical accuracy of the claim that the city existed.
Recently, an archaeologist found what is now generally accepted to be Troy. Does this mean that rest of the story about Troy, including the part that the Greek gods played in the war, are true?