With that combo you can kill any aztec one, so I dont see any problem. They are slow, take 2 pop and are easily out massed.
About chasquis, at least they should get their antiheroe bonus removed, less attack (higher anti merc bonus if needed) and a bit of less speed.
I get the super powerful part. But fantasy units?
Chasquis were a real thing, hell Chasquis are still a real thing among indigenous communities. Sure, they are mostly just glorified Couriers (it’d be like making the Dutch envoy a mainstay of their army), but it’s not like they are fantasy.
Inquisitors can garrison TCs too
I avoid playing Incas now. It is too easy to dominate any supremacy match if I use the chasquis to read the enemy. They are past the point of fixable except in walled in treaty matches. Even there they shine as a team player with the ability to drop forts and war huts in the back of enemy bases.