I'm very sure Paul did not ever claim Christ's suffering was not sufficient to save mankind from our sins.
In this passage, Paul's focus does not seem to be on individual salvation or individual forgiveness of sins. His focus is on Church or Assembly--i.e., the Body of Christ. The Body of Christ is those past, present, future, who are followers of Christ. Paul seems to be saying this is something that could not be accomplished by Christ's suffering on the cross, that his Body must come down off the cross and continue his work of salvation.
Remember, the world has been redeemed. We know repentance for the forgiveness of sins. We know Christ was the final sacrifice and his blood shed for the New Covenant of Redemption and Forgiveness of sins. The way of salvation has been opened to all. There is a new way of living--perhaps the most accurate description is Kingdom Living rather than Worldly Living.
Is being broad-minded about divorce, abortion, homosexuality, transgender operations on children and teens, divorce, "toxic" masculinity, cancel culture, etc. Worldly Living or is it Kingdom Living? To me, all this is more worldly living than it is kingdom living. Shouldn't the Body of Christ, Christ's Assembly shine an equally, if not greater, light on Kingdom Living? What can be done about this?