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
It is done. It is finished. It is consummated.
You say that the doctrine of Purgatory implies a denial of what Jesus said on the cross: It is finished.
If what you say is true, then: I was baptized in 1953. But Jesus said “It is finished” about 28 AD. Therefore, by having me baptized, my parents were asserting that “it” was NOT finished.
To put it another way:
If the notion of the change or growth of people in Purgatory is precluded by Jesus’ words “It is finished,” then the notion of ANY form of change or spiritual growth in people is precluded by Jesus’ words, “It is finished.”
Every time a sinner turns away from sin, he is contradicting Jesus, because turning away from sin implies that the work of salvation is not finished.