What is wrong with 6x6 slides. Bausch & Lomb had a great slide projector that my father had for decades. It disappeared after his death. Maybe he took it with him.
There is absolutely nothing wrong with 6x6 slides, just the automated 6x6 projectors were twice as expensive as the 35 mm (carrousel) projectors.
We had a vast SIMDA set for automatic sound projection mainly used for (commercial-) presentations on business fairs and product launching boots and alike.
A SIMDA projector was about half the price of a Hasselblad 6x6 projector, and as the carousel technique was mandatory for continuous projection, other linear tray 6x6 projector brands, like the very good Rollei, were out of the question.
The SIMDA's were renown for good 4x4cm projection (better luminescence than the Kodak Carousel) and they had Schneider lenses who could very well coop with that format...
The Simda sound sync system (pulse) was very reliable and could continuously operate without the supervision of a projectionist (computer controlled operation was still in its infancy, as well as the internet).
We had a few automatic reverse cassette tape recorders (JVC) providing sync pulses and sound effects: stereo plus a sync track.
And, sometimes we mixed 35mm and 4x4 slides, mainly for panoramic projections by setting two equal projectors side by side (and anchoring them).
Imagine, for the industry, a set two independent images in 4x4 (machinery, diagrams and text) next to each other alternated by a large panoramic view of an ambient setting of these machines...