According to Catholic theology, venial sin ruins one's friendship with God, but it does not sever it completely so that the sinner merits Hell. However, James seems to say that all sin produces death. What is the official harmonization of James with the existence of venial sin?
Then desire conceives and brings forth sin, and when sin reaches maturity it gives birth to death.
You appear to have interpolated an "all" that is not, in fact, present in the verse.
Given that we also have 1 John 5:16-17
If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing as deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly.
one concludes that just as not all sinful desires lead to actual sin, so too not all actual sins lead to death, because they do not all reach "maturity."