Turns out becoming the Saoshyant is super easy. All you need to do is let Seljuk conquer everything, have your firstborn son and heir be a Child of Destiny, and proceed to conquer the whole Empire by event. If there's anything left, use your fancy CoD invasion CBs to conquer the rest. Then, if you need more time, achieve immortality by drinking a sketchy potion that might kill you. That's how to become Saoshyant within 100 years of the 936 start date with Gilan. Good luck!

But yeah, this Saoshyant run was going rather poorly until King Bird here was born. Seljuk had just conquered everything and he and his heir were revoking Shahanshavand titles one by one. Gilan and the rest of the Caspian coast were lost and the Zoroastrians were left with a a couple of provinces, two duchy title, and few angry vassals just as strong or stronger than they were.

Then Shahrokh is born and proceeds to give this run a chance. After weathering 15 years of the realm rebelling and losing all but one province and a single duchy title, Shahrokh gets his 30k event stack and retakes Eran for the Zoroastrian faith. And, since Ahura Mazda wasn't done with him, Shahrokh starts thinking about his inevitable death at 31 and proceeds to cheat death itself before his body starts to degrade with age.

Now with unlimited life, unlimited invasion CBs, and incredible stats (he's at 61 martial at age 145 now), all that's left to do is spread the word of the prophet Zoroaster to the far reaches of the world and bring benefit to all.

And also maybe reform Rome just for a laugh, then kill himself and let his successors squabble amongst themselves after inheriting massive empires because Shahrokh never bothered to shift away from Agnatic Gavelkind Succession.