Tiaral

Am I correct in understanding that the Persians from the older games would be relevant in any setting (for this GM)?

Well, not exactly. But generally, yes. Especially starting with "Reimian Empire" – yes.
No one has written about it in detail. Everyone intuitively embraced this theme.

What is the goal of the game (besides role-playing fun)? How does hero progression work?

Well, role-playing fun.
I don't know how the progression works... Previously, it involved potions, swimming in pools, completing quests, and learning from teachers... Now, it seems like "experience" has been added; you should ask Revan about that.

Are all the heroes on the same side?

In theory, not necessarily, and it has varied. But we generally aim to have them all on the same side; it's more convenient.