The G4 Cube wasn't a callback to the NeXTcube?

Maybe so and NeXTcube totally seems like a nod to the design of Thinking Machines’ CM-2

It was.

Kind of, SGI never had something like Renderman.

They had Blue Moon Rendering Tools (BMRT), though :)

I wasn't aware of it, as per Wikipedia article it looks like the predecessor for Renderman.

The other way around. Renderman is the original, BMRT was mostly Renderman compatible.

Pixar also originally sold hardware, also cube-shaped https://en.m.wikipedia.org/wiki/Pixar_Image_Computer

Depends on exactly what segment of the graphic market, it seems to me that some segments had quite big Sun presence, and early HDTV had the unlikely straggler-survivor with Symbolics.