And that still doesn't explain how they have psyker species in their alliance and somehow, magically, avoid Chaos outbreaks.
Because GW doesn't make Xenos x Chaos stories. That's it. They should be dealing with it. In over 20 years, the only thing we've gotten about how the Tau deal with psykers is a single Patreon AMA by Gav Thorpe where he just shrugs and says, "They let the auxiliars do it."
As far as we know, the Nicassar are as good at dealing with Chaos as the Eldar are, but then again, GW wants you to believe the Tau know nothing about things they have been dealing with for over a thousand years. Like the idea they could not comprehend Titans, Psykers, and Hives despite having fought Orks for millennia before meeting the Imperium, am I to believe they never met a single Gargant before? Not a single Weirdboy? Not a single Ork city with billions of Orks?
They're always at their worst because they're the writers' pets and nothing bad ever happens to them.
Just read it, you can find it online in seconds, and it is one of the best-written novels BL has ever produced. It shows how cruel they can be.
The problem is that IF they move Tau into the next stage, there is no new kid on the block. The best they can do is offer minor AI, chaos and auxiliary rebellion plot lines but nothing too substantial otherwise you upset the balance (without making a 50k with a new Tau race somewhere).
The problem is that over 300 years have passed in-universe, but GW (and the fans) want the Tau to remain static. When they made new Battlesuits for 7th ed, the Farsight books went with "These suits are prototypes" to explain them being around before the First Damocles War, but somehow not present during the War.
That is how you got fans going "Did you know a single Hive World has more people than the Tau Empire?" ignoring that since that early point, the Tau met, and conquered, multiple hives, Sa'cea alone got a population of Trillions.
The Tau have existed for as long as the Necrons out of universe, but as the guy who writes 90% of their novels can't write a plot to save his life, they are nowhere as interesting as the Necrons.