Eh there is only one way to prove this theory. Since both you and OP thing the Vanilla game is easy if you have a good knowlage and skill level and if you min/Max
Why not start as a Zoroastrian count under the Abbasids in the 769 start. Go ahead both you and the Op start as that count in Vanilla and see what happens.
Can you surive long enough to see your religion spread.. or will the Abbasids crush you in mere years ?
EDIT: just noticed you were talking about zoros, not zunists. My post is all about zunists.
Step 1. Quickly conquer adjacent provinces and create k_afghanistan.
Step 2. Swear fealty to the Abbasids.
Step 3. Take Multan, fabricate claims on Baghdad and Cairo and take them. Now should have enough moral authority to reform as soon as you are independent.
Step 4. Join factions to increase council authority. The Caliph gives in to their demands quite often.
Step 5. Start/join a faction to oust the ruler and issue an ultimatum (or wait for someone to do it) during the Shia revolt, for example. The Abbasids can't do a thing without council approval now.
Step 6. Start an independence faction and present an ultimatum when the time is right (another Shia revolt, faction war etc). In my case, I had a support from sultan of Egypt so we both became independent. Abbasids didn't even try to siege down my provinces - they were too busy fightings Shias or rebels (can't recall now).
Bam! You get the first achievement.
Convert to Zoroastrinism, finish off the Abbasids.
Bam! The second achievement.
Then I got bored.
Although I must admit that toying with factions was quite fun. Should I or should I not join this faction? If yes, when? Will we stand a chance against the Caliph? How much faction power should I have? etc.
Shias had k_abyssinia but were holywar'ed recently and I let go of Multan after converting to avoid fighting on 3 fronts (Abbasids were striking back + huge internal faction revolt of zunists).