When Jesus said “It is done,” did that mean that every person who would ever receive his grace had ALREADY received it?
He said "It is finished". It mean that the price required by the Father for sin had been paid.
As for the timing of grace, I thought Catholics liked to talk about God being outside of time, foreknowledge isn't causation or some such. Till man has been conceived, he has no need of grace but as the hymnist put it so eloquently "grace in time of need supply, while I live and when I die." That grace will be supplied in abundance to the believer in the amount required and it is sufficient. 2 Corinthians 12:9
Jesus said it’s done, meaning death has been defeated, the new covenant is here, and if you follow his teachings you will be saved.
I don’t understand it, we don’t need a ton of theologians to discuss this, the salvation of the gospel is simple, a child can do it, and it’s even mentioned that’s how we should come to him.