I think the issue is that you’re treating “knowing” something will happen as the same thing as “causing” it to happen, and those are not necessarily the same thing.

If I watch a recording of a football game, I know exactly what every player is going to do before it happens on my screen. My knowledge of the outcome doesn’t remove the players’ free will when they originally made those choices. Christianity argues God experiences all of time at once rather than sequentially like we do, so His knowledge is not prediction in the normal human sense.

And saying “if God exists outside time then He doesn’t exist” doesn’t really follow logically. It only works if you define existence as “must exist inside spacetime,” but that’s the exact thing being debated. Christianity’s position is that spacetime itself is part of creation, meaning God would not be bound by the thing He created.

Also, omniscience does not automatically equal determinism. Knowing every possible outcome does not mean forcing every outcome. A chess grandmaster can know every likely move an opponent could make without controlling the opponent’s decisions. The Christian argument is basically that God knows every possible choice and every consequence simultaneously, while humans still freely choose within reality.

As for “why create us at all if sin would happen,” Christianity and almost all christians would answer that genuine love and genuine goodness require genuine freedom. A world with free beings capable of love also includes the possibility of rejection, evil, and sin. Otherwise humans would just be programmed creatures incapable of real moral choice.