So here's where I am with my Zoroastrian attempt. Took me a few tries to get here.
I swore fealty to Khiva, and pushed Elective Succession. I then pushed the Karen for Khiva faction, but had to fight a war. Meanwhile, Khiva attacked the Shahi Kingdom in Kabulistan, and took it before my war finished.
Yabguids decided to expand north, and have been involved in a very long and bloody war with Zhetysu, making the northern threat non-existent for 20 years. During this time, I start mass-banishing the Sunni rulers I won, and any other ruler pissed at me.
Saffarids turned their sight onto Merv, while the Abbasids went after that one Tabaristan province. I surrendered that one pretty quickly. I tried to fight off the Saffarids, but despite being evenly matched twice, they won the battles. I surrendered.
10 years later, the Saffarid Shah dies, and his son is caught up in a 4000-man Zoroastrian Revolt in Merv that he cannot defeat. I use this to press all claims on the Merv provinces, avoiding angering the Abbasids in a holy war. (Luckily, they got in a fight with Tulunid Egypt right after this started.)
I have since retaken Merv, and am waiting out the truce timer on Saffarids before I take another section (most likely Khorastan, for Holy Site and Persian/Zoroastrian territories). We're evenly matched, but he beats be by 4-5 ducats/month. I also goofed and gifted away half of my levies when I took Merv (vassalizing Dihistan); Merv is thankfully all Persian/Zoro still, so it will only be a year before it starts producing levies and tax.
Edit: I've also been blessed with a 5000-man strong Ibadi rebellion in the Saffarid's Indian territory.