My experience has been that, unfortunately, accessible sources on early medieval Iran, roughly from the Fall of the Sassanids up to the Seljuk period, are somewhat scarce. I haven't been able to find any podcasts, for example.
There is a plethora of academic sources however. Encyclopedia Iranica is a fantastic, but very academic, online encyclopedia with in-depth information about most things under the sun. "Cambridge History of Iran" volumes 4 and 5 are also great sources for the Crusader Kings time period, and I quite enjoyed a book called "The Eastern Frontier", though that too is very academic. If you're lucky enough to know French or Arabic, wikipedia has links to online editions of Ibn Khordadbeh's chronicle "The Book of Roads and Kingdoms", which was written around 870 (so extremely relevant for CKIII's early bookmark) and describes the Iranian world of that time.
Roughly speaking, however, Iran during CK3's two bookmark dates are in the periods known as the "Iranian Intermezzo" and the "Seljuk Empire". Looking up these two on wikipedia or youtube would probably be a very good starting point, providing a historical overview from which you can then go more in-depth.