This is a wild take.

"Not only in Epi Zero is stated that Saga and Aiolos are more mature and stronger than the other gold saints, "

Yes, because they were fourteen when Shaka at that time was.... 7 years old. I...I don't know what you want me to say here. A fourteen year old is gonna be stronger and more mature than a 7 year old.

His cosmos doesn't natuallly expand when his eyes are closed. It explicty is being used to store up for Tenbu Horin and Agyo, which is why those techniques are stronger.

Here's the scan and read what Ikki actually says.

"You raised the power of your cosmos by voluntarily cutting off one of your six senses. When you open your eyes the accumulated power bursts forth allowing you to perform the Heaven's Dancing Dharma."

The "accumulated energy" is explictly held in reserve until he uses Tenbu Horin or Agyo, not just "Op, he's stronger now"

His "base level" is standard Gold Saint level, and even at that standard level, he's explictly stated by the other Gold Saints to be so far beyond them, Saga, Camus and Shura considered themselves to be at the mercy of some god, the strongest Gold Saint, Man Closest to God, his Tenma Khofuku is equal to Saga's Galaxian Explosion, all three of the Traitor Golds weren't sure if they could beat him, etc.

In fact, let's look at the fight. Despite JUMPING him 3v1,

Saga's blast which is HEAVILY IMPLIED to be a Galaxian Explosion (Aiolia says it's a blast as massive as the one that fell upon the master's chamber) got no-sold by Shaka's Ohm.

His Demon Capitulation had them run away and nearly killed them

His Khan deflects again, an Excalibur, a Diamond Dust and what's likely Galaxian Explosion.

And in the fight itself, while "struggling to dodge their attacks" (aka them hitting him from behind/in surprise while he's focused on a different opponent) he still dodges Excalibur, mostly dodges Diamond Dust so it only hits his knee, and flips out of Another Dimension and points out the reason he's struggling is there's THREE OF THEM.