Is an cosmopolitan empire encroaching in a conservative tribal kingdom an interesting premise? Yes, however - the current presentation of this conflict has left me cold, and I just don't want to touch it.
Both sides have been overly-simplified in theme, while having a bunch of voluminous canon that does little to add to the premise. In fact, most of it narrows the possibility space by moving away from an inspirational constraints to an NPC driven meta-plot where the players are along for the ride.
So, you know what would be more interesting. Exploring how would Orlanthi and Lunars co-exist. Not removing the central conflict from the setting, but letting it sit in the backdrop and instead focusing on a different question.
The possibility space is endless, since it's going to be up to each particular individual to find what that would look like since there are some heavy (trippy) philosophical conflicts that will need the players to explore it - since you (the GM) can't define an explicit moral answer, unless you want a weak ass polemic experience (that goes for game designers too).
Having run a game that was occupied with this question by about the third or fourth session, this is a recipe for a whole lot of fun and the eventual solution we found began with the galaxy-brain and hopefully we can continue it from there.