To be honest, the simplest option would be to use a Duet3 control board.
A "Mini 5" either Wifi or Ethernet should be suitable.
The Duet 3 Mini 5+ is the latest generation controller from Duet3D. The Duet 3 Mini 5+ is a key part of the Duet 3 range of products.
(Just the first link I found, not guaranteed to be the best price).
They will run stepper motors directly.
They can be configured by text files to match the arm mechanics, then positions and speeds can be commanded by XYZ positions in a sequence file or from a web page.
You can also connect an LCD or touchscreen interface for use within the machine.
They are intended for 3D printers but a lot of people use them for other types of machine and robots etc.
Plus they have an excellent support forum to help you with setup and any problems.
Support and Discussion forum for Duet3D Hardware, Firmware, Software, plugins and wider 3d printing/CNC discussion.