Any reason why I can't

use an Arduino Uno and stepper motor shield? And…

As far as I can see the psu is used to provide a 12v source for the stepper motors and 5v for the arduino. If this is correct can I not use the 12v to power the Arduino and steppers and 5v for the opto parts?

Our plan involves a SLAB to power it all and a yaesu ft-857 transceiver for the 2m side

Hey, you can use an Arduino uno and stepper motor shield but i think is necessary to do some changes in the code.

In our configuration, the PSU provides 5V to TP link and to LNA4ALL. The arduino is connected to TP link and is powered via usb from it. The steppers are powered directly from the power source that is 24V from POE. So in the rotator we have 5V (from PSU) and 24V (power source from POE).
In your case you can power the arduino with 5V via usb, opto parts with 5V from arduino and the steppers with power source that is permitted from the stepper motor shield.

Thanks for the reply, Probably best to stick with the original design and make sure it works for me before tinkering