I’ve been considering doing the same, so I am curious how this works out for you.
There has been a bit of discussion on the stepper motors:
Anyone have technical data?
Specifications Overview
Snapmaker Specifications (Google docs)
I don’t recall reading a weight allowance. I remember reading that the rotary module requires new linear modules, but don’t know whether that is for accuracy or power. If power, then you can use the shipping weight of the rotary module (and chuck, though it is one of those small light ones) for an idea of what too-heavy is.