He thinks, He has thought, He will continue to think. He will simultaneously receive several different answers at once, and receives His answer before they arrive. He has been sublimated, but only His body can be bound by the fetters of time, not His thought.
At some point, these thoughts arrive in Mizuki's mind in a stochastic state. It is as if He himself exists in flux at this moment.
If one could clearly know the answers pertaining to the future, that could indeed allow Him to make the correct decisions. But, as a lifeform bound to this dimension, His body and mind cannot transcend the fetters of time. For Mizuki, these hazy, probabilistic forks in the road manifest only after a decision is made, presenting themselves as possibilities in the form of memories. At that time, the option of choosing again will no longer exist, their presence bringing little more than regret or fortuity.
Mizuki rejects such regret and fortuity. He and His powers are nothing special, and from his current perspective, He is nothing more than one of many possibilities for the future, one that violates his original intentions– that obviously cannot be humanity's ideal ending.
However, the decisions that were made that gave rise to the possibility of His birth were presumably set in motion by some version of himself along with the Doctor, with some necessity or rationale in mind. As such, there is no need to treat each other with hostility rather than as friends who made different choices, always leaving each other with some vague ideas to think about afterwards. Mizuki even had a flash of inspiration while playing a game, deciding to give Him a name.
And when Mizuki mentions these things to the Doctor and is able to share the stories that have almost become reality, it has been many long years since they faced the problems that lay beyond the sea together.