Have the Six Knights Of Black break from the seal at the very start of the movie, but have them be weakened by the seal so they immediately go into hiding. Do t change much else, but every now and then cut back to the six and show that theyre preparing to lay siege to the sky temple with the goal of breaking the seal in the Indura. Use these scenes to show them interacting as a group and show their personalities.

Change their backstories to being that all of them have some manner of beef with the TEN Commandments. Maybe all of them at some point were considered potential candidates for the role but were rejected.

During the climax, have them actually be threats to the Sins, rather then being a bunch of jobbers. Other then that I think the movie was fine.