I would have thought up the same modular way, as this saves on the exterior, multiple small fans, and controllers and MB. Bitfury, and ASM have been doing it for awhile, they probably have custom Unit-sizes in house that the public does not know about, in their mines.
Beside BMT upgrade kits, I'm not aware of any other company that successfully reused the chassis, PSUs, fans and controller.
I know that BitFury planed for it but they're currently moving towards immersion cooling in their DCs, so they seem to discard the idea.
In addition, BitFury raspberry pi controller didn't have spare capacity to handle the increase in job creation rate.
We intend to allow quick upgrade between our 3rd and future gens.
We'll probably manufacture the hashing boards closer to the packaging house and ship them by air, while the chassis will be shipped by sea or be manufactured closer to the deployment locations.