Comment by GlenTheMachine

Also, for the type of work I'd do with an arm like this, I'd be more than happy to just have the follower arm. You need a leader arm to do some types of teleoperation or imitation learning, but not really to do reinforcement learning or learn about control theory.

What you do need is an articulated rigid body model that you can import into e.g. NVIDIA Isaac Lab or Gazebo. The availability of a good digital model is a HUGE selling point.