all that exists had to be caused, except the first cause.
Your prior second statement was: "In that case the first cause cannot exist." right after you excepted it. So it was a logical contradiction or non sequitur.
Such a "first cause", as we argued very early on, would be a timeless entity and therefore a changeless entity. What that entails is that all of that "first cause" happened in an instant that took no time. Which further implies that "... on the first day I, your deity, created this... on the second day, that..." - based religions cannot be compatible with the First Cause argument.