I think Vietnam's district rules make for a fun challenge. I'm still unsure if planting forests early is enough of a bonus to compensate for the district placement rules, but it's at least an interesting challenge. I like their other bonuses - "free" encampments means you can get basically all the generals, and them plus the forest movement/combat means you get to play some weird strategic plays in combat.
But yeah, Inca with preserves is a little broken. Basically, hill next to mountain = terrace farm. Flat next to mountain = preserve. I'm a little unsure about the preserves as a whole - like, they're really cool as a new feature, but like, they come really early, and I just don't know how to balance them vs other districts. Like, is a preserve for 2 breathtaking tiles worth it? Or should I just build a campus/commerce hub/holy site/etc... as normal? But I definitely like how you finally have a real reason to look for appeal, and make for some big yield tiles if you get the right setup.