I earlier said that an FPGA would work fine......
Because there was one of these right in front of me:
https://www.aliexpress.com/item/EP4...-Board-Core-Kit-free-shipping/1675716755.html
and I was learning about Verilog 'generate' syntax, I typed up a controller for manymany servo-motors.
- Receives data from from USB in funky-but-straightforward format using one of these
https://www.sparkfun.com/products/7841
- Controls as many servos as the FPGA has available pins (less the USB, clock etc inputs)
- Synchronous-serial output to other FPGAs for I/O expansion
The board above has enough I/O for 56 outputs, and logic resources are 21% used. Adding something like this :
**broken link removed**
... would allow adding 90+(?) outputs for about 25¢ US each.
(Note that the latest version 14.0 of Altera tools has dropped support for Cyclone II: you'd need version 13.0 or earlier.)
All in all, this would make your hardware-only cost about 12 boards@$30 = $360
Wiring, carrier boards and I/O protection (a single level-translator 3.3-5V on each output) would add at least $150
(..... says the guy with no money at risk.)
This is only one way to do it, but the low hardware-outlay and wiring simplicity deserve consideration.