“...and the plain text says that something is Missing (lacking) in Christs Suffering and it must be added to. And we must add our own work.”
Your understanding is in direct contradiction to Hebrews 9:12 “With His own blood - not the blood of goats and calves - He entered the Most Holy Place once for all time and secured our redemption forever.”
Jesus’ sacrifice is not lacking in any way. You can try to add your pitiful good works on top of what Jesus did, but His work is the greatest of all and is sufficient.
What does the plain text say?