Where’s the point where you figure out how to stack chiplets perpendicular to a backplane instead of doing lithography 300 times on the same chip?

As others here have pointed out -- you don't need to do lithography 300 times. The big breakthrough of 3D NAND is depositing alternating layers of films to build most of the layers.

This is 3 stacks of >100 layers.

That's kind of what we're doing already, although the stacking is parallel, not perpendicular. A lot of the innovation is in how the dies are tied together, cf. https://www.anandtech.com/show/9520/toshiba-brings-throughsi...

This is an interesting testament to manufacturing reliability - that they can go to so many layers and still achieve good yield.