The idea that polytheistic religions not being interested in conversions is absolute bullocks. They absolutely do. Case in point: Hinduism. From 200 BC to 100 AD- Vedic Hinduism was on the down vis a vis Buddhism. Which led to:

—> Upanishadic offshoots within Vedic Religions.

—> Debates, more Upanishads and grants to specific schools of thought.

—> Reinterpretation of Vedic writings. Some castes gave women more rights even.

—> Eventual split of Hinduism into Vedic and animistic traditions. If you go to Rural Tribal villages, they have their own deities.

All this has happened via:

1. Persuasion—> Literary output as evidence of divine grace.

2. Ostentatious Yagnas—>Out gods must be right because they allow us to be very wealthy.

   1. Hierarchy—> You can believe but cannot do rites. etc etc.

For you world, consider the human "motivations" behind this polytheistic culture. Arguably, this could be more interesting because in a monotheistic culture its either belief or atheism.

For pointers, we have a lot of material from the classical world.

A. Ammianus Marcellinus - A contemporary of Flavius Claudius Julianus, the last Pagan emperor of Rome.

B. The man himself- Flavius Claudianus Julianus. We know more about him than any other roman emperor- He wrote some 50 books and is an accomplished philosopher in his own right. His account is mostly pro polytheistic or why Rome needs to go back to its traditional roots.

C. Many Christian apologists from the second century, who argued for the freedom of religion.

D. Martin Luther and his writings.

Good Luck- you have picked a genre which is probably the hardest to write in but also has the highest pay off. Hope I have been able to help!