Is this optocoupler-based circuit design reliable?
The stepper driver input is pulled up internally by R4.  At 12 V and 4.7 k&Omega;, it takes only 2.6 mA to pull the input solidly low.  That's well within the range of common opto-couplers.  You can simply connect the opto output between the stepper driver input and ground.  R2, R3, and Q1 are just silly.

Other than that, work backwards to make sure you are driving the LED in the opto with sufficient current.  That needs to be at least 2.6 mA divided by the current transfer ratio.  It's good to leave some margin, so you probably want to have the nominal current be maybe 50% higher than the minimum necessary value.

No, you don't get to just ignore the current transfer ratio.