To be blunt, I regard Jesus mythicism as pure pseudo-historical dogma by a few cranky atheists. For them, the non-existence of Jesus is somewhere between the holy grail of unbelief (i.e. wouldn’t it be good if we could prove Jesus never existed?) and mental masturbation (i.e the sheer selfishly indulgent pleasure of thinking about it). Jesus mythicism is an unscholarly conspiracy theory and its adherents should be treated in the same way we treat climate change deniers: A polite chuckle, sip your martini, then avoid eye-contact, and slowly walk away.
I sit on the board of the Journal for the Study of the Historical Jesus, a board filled with people of all faiths and none, Evangelical Christians, mainline Christians, former Christians, Agnostics, Jews, and Atheists. Now we disagree on A LOT, but we ALL AGREE that Jesus existed. In fact, we had a great discussion on Jesus mythicism at our last board meeting. Larry Hurtado made a great point that Jesus mythicism is like a weird monster that rears its head about every fifty years and needs to be periodically and ritually destroyed. Maybe they’ll do an issue on it one day.
Otherwise, see my older piece: Yes, Jesus existed but relax, you can still be an atheist if you want to.