Everything else had to wait until he restored that completely ruined pair of Mangekyō Sharingan.
As for why he didn't fuse Fugaku Uchiha's Mangekyō with the pair in Karin's eye sockets, Shinji naturally had his reasons.
Fugaku Uchiha's Mangekyō was a safety net for him.
If Sasuke couldn't successfully obtain Itachi's Mangekyō in the future, Fugaku Uchiha's pair could serve as a backup and ensure that Sasuke could fuse them into an Eternal Mangekyō Sharingan containing Ōtsutsuki Indra's Chakra.
So unless he had no other choice, he wouldn't use Fugaku Uchiha's Mangekyō lightly.
"No rush. I'm still young. I can take my time and plan ahead!"
Shinji calmed himself.
The Power of All Things was an extremely advanced power in the Shinobi World. It was the threshold to reaching Six Paths level, and in the past thousand years, only a handful of people in the Shinobi World had mastered it.
Therefore,
this wasn't something he could rush. He needed the right time, place, and people.
As things stood, Shinji had already started from a very high point. On top of that, Madara Uchiha had already blazed a trail he could use as a reference.
And this generation's reincarnations of Ōtsutsuki Indra and Ōtsutsuki Ashura were both right by his side.
As long as he dared to make bold assumptions, carefully tested them, and explored little by little, drawing out the Power of All Things and striving for the Rinnegan would be well within reach.
He closed his eyes, cleared his mind of all distractions, and began to focus, carefully sorting through his plans.
First, he would exchange Chakra with Karin regularly and enter a shallow state of Yin-Yang union.
This would not only improve the quality of both their Chakra, but also give them a chance to explore how to