And you forgot that Tenma fought against "himself" in the Mukai....
Dark Sisyphos wasn't fresh and new at all.
Dark Sisyphos didn't interact in the Gaiden with the same character than in the main story, where he didn't fight at all except for the arrow he shot to Sasha, in the Gaiden it was a different role. And he was a guest star too, he wasn't the main antagonist (there was also a "new" enemy with the Centaur Specter).
Maybe you prefer the classic "evil enemy trying to screw things around". I liked this very much because it had good character development, not like El Cid's Gaiden that was hollow as a bone.
Sisyphos did - indeed - interact with his inner doubts in both stories. The part in the Mukai was are speaking about was consisting of 2 full chapters with only him and Sasha.
Kagaho was less important in the gaiden than Asmita, his antithesis Atavaka and the sidekick-witness-simple mortal Ahimsa.
"Centaur" Specter is the only new enemy in Sisyphos Gaiden and his presence is really minimal.
For me, these gaiden are mainly here to develop the backstories and personalites of the Gold Saints
Chapter 3 of the gaiden put Sisyphos in a double already-seen situation, with chapter 2 and with the Mukai.
Saint Seiya The Lost Canvas Gaiden Volume 10 on sale 20th June.
Enfin tant mieux, on aura le schéma du spectre aux Centaures plus vite
Tiens, un nouveau sagiglaire ?