So I've gone through 1.5 pairs of Index controllers now, and I'm pretty sure the thumbstrick drift issue is connected with how much you press in the thumbsticks. If you use gestures or jump a lot in Vrchat, you should think about binding those functions to the A buttons, especially if you're having problems with the thumbstick capacitive sensors being "sticky". That usually foreshadows or comes hand in hand with drifting in my experience.