The main religion of the empire I want to focus on worships the sun.
There is supposed to be a southern realm. Not a unified realm, more of a cultural group. That the empire often wars with, but also lots of exchange and trade going on. At some point, these people fucked the empire over by pulling out some holy fire religion.
My problem is simply that it's thematically too close. Yes, the sun as a celestial body and the element of fire are different. But both are fire, both are light. It's too thematically similar. Could offer some fun religious discussions and interactions in-universe, still. My other problem with that is, it feels too pagan. This southern realm is supposed to be based on renaissance Italy, cultures around the Adriatic Sea, Spain, orthodox Greece and such.
Maybe something with saint worship, but no idea where their powers are supposed to come from.