@Paul_B THANKS .
Your suggestion takes me back to a post I started months ago, now closed.
Here's the link :
https://forum.arduino.cc/t/creating-the-bmw-kinetic-sculpture/670206/280
It's very long, 280 answers. But if you scroll to the last post you'll see a post by WAWA who designed and made boards with TPIC6B595 which could control 48 steppers.
He was so kind to send me 2 of his boards. Unfortunately, dumb me, never understood how to use them. I guess I'll just have go back to those boards and see if I can find out how to use them. If you look at the picture of the boards you'll see they are ready to use and professionally made.
One last question pls. Do you think there will be no synch problems ? The patterns require millimetric synching.