Actually...
Shisui was not a chatterbox!
It was just that he could feel the inner pain of the Itachi before him. Even though this wasn't the Itachi from his own world, Shisui still felt a deep sense of empathy.
Therefore,
even though Itachi had only asked a single question, Shisui explained the entire process of Uchiha Jin's rise: how he seized power step-by-step, built up his forces, won over the Ninja Clans, and ultimately founded Neo Konoha, crushing Old Konoha.
His reason for doing so was actually very simple!
Shisui simply wanted this tormented Itachi to truly see a glimmer of hope in his desperate situation.
More importantly...
As someone who had also awakened the Mangekyo, he knew better than anyone that this power came with a price!
The thoughts in one's mind upon awakening the Mangekyo would be distorted and alienated by its power, gradually turning into an obsession.
Regardless of whether this obsession was right or wrong, and regardless of whether it was still appropriate as times changed!
In short...
Unless a stronger person emerged or someone with an incredibly powerful mindset appeared, it would be very difficult to change that person's state of mind.
In the past, Uchiha Madara had Senju Hashirama to keep him in check!
Hashirama suppressed him when he awakened his Mangekyo, and suppressed him again when he awakened the Eternal Mangekyo, repeatedly keeping Uchiha Madara completely in check.
In the end,
tricked by Black Zetsu, Uchiha Madara awakened the Rinnegan without Senju Hashirama around, and thus, Uchiha Madara became a major villain!
There were many other similar examples!
Shisui himself and the Itachi of the original world had Uchiha Jin to keep them in check, so over time, their mindsets gradually stabilized.
Obito had no one to keep him in check. The sudden shift in his mindset led him to kill the Minato couple!
When Fugaku awakened his Mangekyo, he was the clan leader and was completely suffocated by the pressure from the Konoha Higher-ups. Thus, the Mangekyo reinforced his mindset at the time, transforming him from the initial Wicked Eye Fugaku into the later Uchiha the Great Coward!
The Itachi before him was the same.
He had awakened his Mangekyo for Shisui's sake. The task Shisui had entrusted to him back then was to watch over the Uchiha to prevent a rebellion and protect the peace of the Village!
Itachi's Mangekyo was awakened for this purpose, so it naturally reinforced this point instantly.
Following that,
Itachi became extremely stubborn. The alienating power of the Mangekyo left him with only this single mission for his entire life.
At first, he tried to make a change, but after failing to think of any way, the Mangekyo's alienation of his mindset gradually led him down a distorted path.
Loyalty to the Konoha Higher-ups?
Heh, not at all!
After becoming a Rogue Ninja, he was nominally a Konoha spy, but he never provided any intelligence to Konoha. In the end, it was Jiraiya who traded his own life to obtain information on the Akatsuki Organization.
Why?
He had no loyalty to Konoha in the first place. The power of the Mangekyo only ensured that the Uchiha would not rebel and that Konoha remained peaceful.
Now that the Uchiha Clan was gone, you tell me: had peace arrived, and had the Uchiha Clan's rebellion been resolved?
Once this matter was over, the effect of the Mangekyo naturally dissipated instantly!
Subsequently,
with the wish from when he awakened the Mangekyo fulfilled, and Itachi immersed in immense grief, he lost his last chance to find himself and could only be led by the nose by the Mangekyo.
Itachi's Mangekyo was actually also bewildered!
What now?
The host's wish has already been fulfilled!
And the host seemed to have no other thoughts about his life, nor had he thought about what to do in the future.
So, there was nothing it could do!
With nowhere to direct its alienating and distorting power, the Mangekyo began to dig into the host's mind and found a rather strong trait from the scrap heap: a brother complex.
Let's go with this.
Since Uchiha Itachi's life had lost its meaning anyway, it couldn't just let the host die immediately or continue living as a walking corpse, right?
Ultimately,
after settling the two wishes of Konoha's peace and the Uchiha rebellion, guided by the distorted Dōjutsu Power of the Mangekyo, Uchiha Itachi embarked on the path of training Sasuke to make him strong.
Of course,
it couldn't affect Konoha's peace, because that was the host's original wish, and this pair of Mangekyo had been awakened precisely because of those two wishes.
Therefore,
Itachi embarked on a path of helping Sasuke grow stronger while simultaneously preventing Sasuke from becoming hostile toward Konoha, ultimately leading Sasuke to awaken the Eternal Mangekyo Sharingan.
With all wishes completely fulfilled, Itachi still did not find himself, and his Mangekyo's mission came to an end.
At the very end, Itachi simply died!
Shisui knew this all too well. Especially just now, outside the Tsukuyomi space, when Itachi had vented those words, Shisui instantly realized that Itachi could still be saved!
At the very least, this pair of Mangekyo definitely had something to do with him.
Otherwise...
Itachi, who had awakened the Mangekyo, would not show such humanity!
This was also why Shisui had spoken those words without waiting for Itachi to ask; he wanted to help Itachi find a new path to peace.
As long as Itachi did not massacre the Uchiha Clan, he would absolutely not lose himself. Plus, Shisui's own strength was greater than Itachi's, and Itachi's Mangekyo had been awakened for his sake.
The probability of helping Itachi find his true self and resisting the alienating and distorting power of the Mangekyo was actually very high.
And indeed, it was so!
As Shisui detailed Uchiha Jin's approach, Uchiha Itachi, who was naturally a highly intelligent person, began to automatically calculate the feasibility of this method.
Simply put, it was a simulation!
The Mangekyo in Itachi's eyes spun frantically as he began to run a rapid simulation of this path, based on the route Shisui had described and his own understanding of Konoha's darkness.
A few minutes later,
even the alienated and distorted power of the Mangekyo had to admit that this path was highly feasible. Combined with Shisui's appearance, Itachi's deadened self began to slowly revive...
With these two factors combined, Itachi finally nodded.
The Mangekyo in his eyes stopped spinning, and he began to look squarely at the familiar stranger before him.
After a long while,
Itachi finally spoke.
"The path you described indeed has high feasibility."
"The damage to Konoha—controllable!"
"Our entire Uchiha Clan would absolutely never have thought of this kind of solution. Even with my understanding of the Nara Clan, they wouldn't have been able to come up with this kind of method either."
"Shisui..."
"I believe you now!"
"You are indeed not the Shisui of this world. He could never have laid out such a feasible path, otherwise, he wouldn't have died in despair back then!"
"But it really is hard to imagine that parallel worlds actually exist."
"In your world, the Uchiha Clan is truly fortunate!"
"But a contemporary of Kakashi, the child of Uncle Tianyi and Aunt Keiko?"
"Uchiha Jin, I remember now!"
"The Uchiha Jin of my world only awakened the two-tomoe Sharingan and died in the Third Great Ninja War."
"What a pity!"
"If we had known that Lord Jin's thinking was so incredible, the Uchiha should have protected Lord Jin's life back then, even if it meant sacrificing everything."
"What a pity!"
"..."